Add them as background images in the table headers.
Table JSON
[
  {
    "type": "ia.display.table",
    "version": 0,
    "props": {
      "data": [
        {
          "city": {
            "value": "Folsom",
            "editable": true,
            "style": {
              "backgroundColor": "#F7901D",
              "classes": "some-class"
            },
            "align": "center",
            "justify": "left"
          },
          "country": "United States",
          "population": 77271
        },
        {
          "city": "Helsinki",
          "country": "Finland",
          "population": 635591
        },
        {
          "city": "Jakarta",
          "country": "Indonesia",
          "population": 10187595
        },
        {
          "city": "Madrid",
          "country": "Spain",
          "population": 3233527
        },
        {
          "city": "Prague",
          "country": "Czech Republic",
          "population": 1241664
        },
        {
          "city": "San Diego",
          "country": "United States",
          "population": 1406630
        },
        {
          "city": "San Francisco",
          "country": "United States",
          "population": 884363
        },
        {
          "city": "Shanghai",
          "country": "China",
          "population": 24153000
        },
        {
          "city": "Tokyo",
          "country": "Japan",
          "population": 13617000
        },
        {
          "city": "Washington, DC",
          "country": "United States",
          "population": 658893
        },
        {
          "city": "Wellington",
          "country": "New Zealand",
          "population": 405000
        },
        {
          "city": "Delhi",
          "country": "India",
          "population": 11034555
        },
        {
          "city": "Dhaka",
          "country": "Bangladesh",
          "population": 14399000
        },
        {
          "city": "Lagos",
          "country": "Nigeria",
          "population": 16060303
        },
        {
          "city": "Karachi",
          "country": "Pakistan",
          "population": 14910352
        },
        {
          "city": "Istanbul",
          "country": "Turkey",
          "population": 14025000
        },
        {
          "city": "Cairo",
          "country": "Egypt",
          "population": 10230350
        },
        {
          "city": "Mexico City",
          "country": "Mexico",
          "population": 8974724
        },
        {
          "city": "London",
          "country": "United Kingdom",
          "population": 8825001
        },
        {
          "city": "New York City",
          "country": "United States",
          "population": 8622698
        },
        {
          "city": "Tehran",
          "country": "Iran",
          "population": 8154051
        },
        {
          "city": "Bogota",
          "country": "Colombia",
          "population": 7878783
        },
        {
          "city": "Rio de Janeiro",
          "country": "Brazil",
          "population": 6429923
        },
        {
          "city": "Riyadh",
          "country": "Saudi Arabia",
          "population": 5676621
        },
        {
          "city": "Singapore",
          "country": "Singapore",
          "population": 5535000
        },
        {
          "city": "Saint Petersburg",
          "country": "Russia",
          "population": 5191690
        },
        {
          "city": "Sydney",
          "country": "Australia",
          "population": 208374
        },
        {
          "city": "Abidjan",
          "country": "Ivory Coast",
          "population": 4765000
        },
        {
          "city": "Dar es Salaam",
          "country": "Tanzania",
          "population": 4364541
        },
        {
          "city": "Wellington",
          "country": "New Zealand",
          "population": 405000
        },
        {
          "city": "Los Angeles",
          "country": "United States",
          "population": 3884307
        },
        {
          "city": "Berlin",
          "country": "Germany",
          "population": 3517424
        },
        {
          "city": "Jeddah",
          "country": "Saudi Arabia",
          "population": 3456259
        },
        {
          "city": "Kabul",
          "country": "Afghanistan",
          "population": 3414100
        },
        {
          "city": "Mashhad",
          "country": "Iran",
          "population": 3001184
        },
        {
          "city": "Milan",
          "country": "Italy",
          "population": 1359905
        },
        {
          "city": "Kiev",
          "country": "Ukraine",
          "population": 2908703
        },
        {
          "city": "Rome",
          "country": "Italy",
          "population": 2877215
        },
        {
          "city": "Chicago",
          "country": "United States",
          "population": 2695598
        },
        {
          "city": "Osaka",
          "country": "Japan",
          "population": 2691742
        },
        {
          "city": "Bandung",
          "country": "Indonesia",
          "population": 2575478
        },
        {
          "city": "Managua",
          "country": "Nicaragua",
          "population": 2560789
        },
        {
          "city": "Paris",
          "country": "France",
          "population": 2229621
        },
        {
          "city": "Shiraz",
          "country": "Iran",
          "population": 1869001
        },
        {
          "city": "Manila",
          "country": "Philippines",
          "population": 1780148
        },
        {
          "city": "Montreal",
          "country": "Canada",
          "population": 1649519
        },
        {
          "city": "Guadalajara",
          "country": "Mexico",
          "population": 1495189
        },
        {
          "city": "Dallas",
          "country": "United States",
          "population": 1317929
        },
        {
          "city": "Yerevan",
          "country": "Armenia",
          "population": 1060138
        },
        {
          "city": "Tunis",
          "country": "Tunisia",
          "population": 1056247
        }
      ],
      "enableHeaderGroups": true,
      "columns": [
        {
          "field": "city",
          "visible": true,
          "editable": false,
          "render": "auto",
          "justify": "left",
          "align": "center",
          "resizable": true,
          "sortable": true,
          "sort": "none",
          "filter": {
            "enabled": false,
            "visible": "on-hover",
            "string": {
              "condition": "",
              "value": ""
            },
            "number": {
              "condition": "",
              "value": ""
            },
            "boolean": {
              "condition": ""
            },
            "date": {
              "condition": "",
              "value": ""
            }
          },
          "viewPath": "",
          "viewParams": {},
          "boolean": "checkbox",
          "number": "value",
          "progressBar": {
            "max": 100,
            "min": 0,
            "bar": {
              "color": "",
              "style": {
                "classes": ""
              }
            },
            "track": {
              "color": "",
              "style": {
                "classes": ""
              }
            },
            "value": {
              "enabled": true,
              "format": "0,0.##",
              "justify": "center",
              "style": {
                "classes": ""
              }
            }
          },
          "toggleSwitch": {
            "color": {
              "selected": "",
              "unselected": ""
            }
          },
          "nullFormat": {
            "includeNullStrings": false,
            "strict": false,
            "nullFormatValue": ""
          },
          "numberFormat": "0,0.##",
          "dateFormat": "MM/DD/YYYY",
          "width": "",
          "strictWidth": false,
          "style": {
            "classes": ""
          },
          "header": {
            "title": "",
            "justify": "left",
            "align": "top",
            "style": {
              "classes": "",
              "backgroundImage": "url(\"/system/images/Builtin/icons/48/logic_and.png\")",
              "height": "60px",
              "backgroundRepeat": "no-repeat",
              "backgroundPosition": "bottom"
            }
          },
          "footer": {
            "title": "",
            "justify": "left",
            "align": "center",
            "style": {
              "classes": ""
            }
          }
        },
        {
          "field": "country",
          "visible": true,
          "editable": false,
          "render": "auto",
          "justify": "auto",
          "align": "center",
          "resizable": true,
          "sortable": true,
          "sort": "none",
          "filter": {
            "enabled": false,
            "visible": "on-hover",
            "string": {
              "condition": "",
              "value": ""
            },
            "number": {
              "condition": "",
              "value": ""
            },
            "boolean": {
              "condition": ""
            },
            "date": {
              "condition": "",
              "value": ""
            }
          },
          "viewPath": "",
          "viewParams": {},
          "boolean": "checkbox",
          "number": "value",
          "progressBar": {
            "max": 100,
            "min": 0,
            "bar": {
              "color": "",
              "style": {
                "classes": ""
              }
            },
            "track": {
              "color": "",
              "style": {
                "classes": ""
              }
            },
            "value": {
              "enabled": true,
              "format": "0,0.##",
              "justify": "center",
              "style": {
                "classes": ""
              }
            }
          },
          "toggleSwitch": {
            "color": {
              "selected": "",
              "unselected": ""
            }
          },
          "nullFormat": {
            "includeNullStrings": false,
            "strict": false,
            "nullFormatValue": ""
          },
          "numberFormat": "0,0.##",
          "dateFormat": "MM/DD/YYYY",
          "width": "",
          "strictWidth": false,
          "style": {
            "classes": ""
          },
          "header": {
            "title": "",
            "justify": "left",
            "align": "top",
            "style": {
              "classes": "",
              "backgroundRepeat": "no-repeat",
              "backgroundImage": "url(\"/system/images/Builtin/icons/48/oszillograph.png\")",
              "height": "60px",
              "backgroundPosition": "bottom"
            }
          },
          "footer": {
            "title": "",
            "justify": "left",
            "align": "center",
            "style": {
              "classes": ""
            }
          }
        },
        {
          "field": "population",
          "visible": true,
          "editable": false,
          "render": "auto",
          "justify": "auto",
          "align": "center",
          "resizable": true,
          "sortable": true,
          "sort": "none",
          "filter": {
            "enabled": false,
            "visible": "on-hover",
            "string": {
              "condition": "",
              "value": ""
            },
            "number": {
              "condition": "",
              "value": ""
            },
            "boolean": {
              "condition": ""
            },
            "date": {
              "condition": "",
              "value": ""
            }
          },
          "viewPath": "",
          "viewParams": {},
          "boolean": "checkbox",
          "number": "value",
          "progressBar": {
            "max": 100,
            "min": 0,
            "bar": {
              "color": "",
              "style": {
                "classes": ""
              }
            },
            "track": {
              "color": "",
              "style": {
                "classes": ""
              }
            },
            "value": {
              "enabled": true,
              "format": "0,0.##",
              "justify": "center",
              "style": {
                "classes": ""
              }
            }
          },
          "toggleSwitch": {
            "color": {
              "selected": "",
              "unselected": ""
            }
          },
          "nullFormat": {
            "includeNullStrings": false,
            "strict": false,
            "nullFormatValue": ""
          },
          "numberFormat": "0,0.##",
          "dateFormat": "MM/DD/YYYY",
          "width": "",
          "strictWidth": false,
          "style": {
            "classes": ""
          },
          "header": {
            "title": "",
            "justify": "left",
            "align": "top",
            "style": {
              "classes": "",
              "backgroundRepeat": "no-repeat",
              "backgroundImage": "url(\"/system/images/Builtin/icons/48/wrench.png\")",
              "height": "60px",
              "backgroundPosition": "bottom"
            }
          },
          "footer": {
            "title": "",
            "justify": "left",
            "align": "center",
            "style": {
              "classes": ""
            }
          }
        }
      ]
    },
    "meta": {
      "name": "Table_0"
    },
    "position": {
      "basis": "299px"
    },
    "custom": {}
  }
]