You could make it download avatars of a fairly standard size (say, 128x128) only in the chat with that person, and you could do it from external servers by just tracking the image url so they don't even have to upload to RTB. Just have it check the size of the image when they try to set it, so they can't set larger/smaller images, and it could download the image when you open the chat with them (until it's downloaded, just show a 'blank/no avatar' image, or a loading image).
Of course that's useless and superfluous, but it would be nice to have.