I was going to say "but what about the fact that every service has its own size for user icons" and then I re-read this and saw that point 4 says it might, perhaps, have a mode where it returns a list of multiple images/formats/sizes. And thinking about that makes me remember LiveJournal, which had the wonderful feature of letting you have multiple icons and the ability to pick one for each post and/or reply; nobody's ever duplicated this and I really miss the layer of meaning it added on top of plain text.