Making the thumbnailing more bandwidth-friendly

Sorry in advance for bikeshedding. :)

If you look at the flickr image API, the 500px image API, and probably others, you'll notice that they tend to offer a list of images -- this way the client application can generally pick the most bandwidth-friendly size for the purposes needed.

Everything else I could think of possibly doing to the image API is something that is more hardcore, probably specific to a given service, and something that can be just added as an extra key (e.g. EXIF information, IPTC tags, user information, etc).  I just think that just about any image serving service tends to offer a set of resized images and that this way, you don't have each service built upon this will end up with their own clunky way to offer the multiple thumbnail sizes.