You could do the same thing with PNGs and different thunk types. Although in both cases you run a risk that some paranoid developer might filter out unexpected

You could do the same thing with PNGs and different thunk types. Although in both cases you run a risk that some paranoid developer might filter out unexpected thunk types or additional data so in both cases it would be best to put the data in the image payload. The other consideration is that Tumblr was always very “creator” oriented and while they might produce thumbnails of various sizes the original image is still available and not mangled by resizing algorithms. Other free image hosts are going to crush that image down the maximum amount tolerable to the human eye. Google even does that for paid photo hosting.