The one that you're using now is acceptable, I'm just speaking in general. A sig pic is just a little calling card. They shouldn't be obtrusive and tall sig-pics are obtrusive. It's better if they're wider than taller because aesthetically it blends with the layout of a forum better and on a practical and more important note, people have to scroll less to read the forum. When someone has a tall sig-pic, people have to scroll a lot more to read the forum. See?
I think it's because it's dynamic. Fotki seems to have some security measure in place that block images from being displayed inline.
