I strongly disagree with the design choice, but here's how to do it:

You have to set the following styles on your main flex container:

{
"classes": "",
"backgroundColor": "#FFFFFF",
"backgroundRepeat": "no-repeat",
"overflow": "auto",
"backgroundImage": "url(\"/system/images/foo.png\")",
"backgroundPosition": "bottom",
"backgroundSize": "cover"
}
Play around with the Position and Size properties as needed.
Additionally, any components you want the background to be visible through need the style property
backgroundColor: transparent
By the way, from your first picture it seems that you were just trying to drag and drop the image into the flex container, which adds it as a child. That's why it was getting pushed around by the other components.