Somewhere along the line of many forum upgrades and server changes, we experienced a bug that was not obvious for a while. All of the avatar images shrunk and the quality was really degraded. Whenever I tried uploading a new image, it would shrink too, even though it was not too big. Some of the fixes Ken has put in place lately appears to have fixed the issue. So upload your avatar again. 100x100 is the max dimensions. I'm going to work on getting our old custom avatars reloaded as well.