In the ideal future, FLIF [0] would become a standard, universally supported image and animation format. Almost any subset of a FLIF file is a valid, lower-resolution FLIF file. This would allow the browser - or the user - to determine how much data could be downloaded, and to display the best-quality images possible with that data. If more bandwidth or time became available, more of the image could be downloaded. The server would only have one asset per image. Nice and simple.

[0] http://flif.info/