conclusion (although probably implemented after the first public alpha):
avatars will be uploaded at whatever-you-wish and resized to 48x48 as base size. blahblah will resize avatars to different sizes as needed. it works rather well for upcoming.org, where sometimes avatars are as small as 8x8 pixels.
although i don't plan to use that small avatars in blahblah ... though if it evolves into an even more chatty application..... .... forget that