The avatar/profile picture stuff on Twitter/Socialbook isn't necessarily a bad idea, just more complicated than what I have currently. Right now, I only have to store simple, small images. If I were to implement being able to enlarge avatar pictures, I would have to store multiple versions of it at various qualities, spend time during the upload process making each version, and provide some relationship in the database between the user and their multiple versions.

It's not impossible, just more complicated.

Here's what I can do: I've increased the quality of the still images to 100%. This means that if you re-upload the avatar, your picture of dear Jean-Luc will no longer be mottled by compression algorithms.