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.