Images in image management are actually automatically hosted by the platform's webserver, so one option would be to set a data key to a fully qualified HTTP path to the image:
data['url']='http://localhost:8088/system/images/bt.jpeg'
In the below image, the top image was created by dragging and dropping from image management, and is totally static. The bottom image is using the url key populated by the script above.
NB: I already updated the image at this path since dragging and dropping the top image in.
Now the preview in the designer is wrong, because it's using a cached version of the image:
But the actual exported report is correctly updated:
