The current code for avatars in threads is:
(Using mine as an example)
<img style="border:1px solid #C4C4C4;margin-left:45px;padding:2px;width:80px;" src="customavatars/avatar6329_6.gif" alt="aronpm's Avatar" title="aronpm's Avatar">
The width:80px part is an issue. I demonstrated this by putting up a 1x80 avatar, which was then rendered in a thread as 80x640. This has potential for abuse so I think that should be fixed. I don't think it's necessary for that part to be there; if an avatar is uploaded successfully it is already 80x80 or smaller so it can't stretch the page any more than width:80px can. Also, avatars with a width smaller than 80 pixels become stretched by this.