Technically the size limits are 192x192, so it is being scaled slightly. That might be a cause, but most browsers can scale images down quite well. You could always try grabbing the original and scaling it down to 192x192 and see if that helps?... Although, when I post on the forums this avatar looks blurry to me and I can't figure out if it's my eyes or the fact that the upload was 200x198 verses 200x200. :x
Incidentally, not-square avatars are displayed as is, we don't stretch them to fit. Or at least we don't now, I fixed that bug
Also the reason I came into this thread: just got told I have a job interview tomorrow, yay!