As I mentioned before, I don't see any problems with avatars, regardless of where they are uploaded. I tested my avatar by uploading it to ImageShack and some other image hosting services, and I also tried to upload it directly to this forum (wherever is that server that Alwil is using), and I still see avatars in forum.
Mayeb it really has something to do with image extensions (file types). I never had problems with Gifs, but maybe some problems occurs when we use some other file type or something... I don't know...
However I just did that for testing purposes, but usually I have my web filtering disabled (in Kerio), since I use some third party tools...
Cheers !