It may also be the forum theme that you use. The teal and black that I use right now for some reason reshapes avatars to make them all be square, so what on the default theme looks like whatever shape it's supposed to be, this theme distorts it by forcing it to be a certain size... It's kind of irksome.

So that said, if you want it to look good on all themes perhaps a perfectly square-shaped size is what you need. 100x100 is my current size.