First, the avatar settings are set to a maximum of 150 pixels by 150 pixels and no, sorry, this won't be expanded (for those who are asking). Wider than 150 distorts the forums too much.
Second, if you are having problems with your avatar looking stretched out (like with Mark's BGE avatar was) I figured out what's causing it and have (da dum!) a solution.
In your own profile settings in your user control panel go to your avatar settings. Near the bottom of the form you will see a field for "avatar size". There will be two boxes (height and width). Any values you put into those boxes will force the forum to assume your avatar is that size, even if it's not, so that can distort your particular avatar if you use the wrong values. For instance, Mark's avatar was actually 57 pixels by 120 pixels... those fields were set to 150 x 150 so it was trying to scale it to that size and that's not an even ratio so it ended up distorting it and stretching it out of shape.
You have two solutions... first, put the *exact* dimensions of your avatar in that box (or an even ratio of them). You can find this info by right clicking your avatar and looking at the properties. If you have a small avatar that you want automatically scaled up (or vice versa) this can be handy... for instance, if you have an avatar that is 40 pixels high and 75 wide and you want to double it in size, if you put in 80x150 it should automatically scale it up and still keep it in proportion without stretching it out (and vice versa).
Your second option is to leave the fields blank. Completely blank. If you do that, the board will try to detect what size your avatar is and act accordingly (which means basically it will leave it alone in most cases and it might scale it down to fit in the limits if it's too large).
Questions, as always, should be posted in the support section and we'll try to figure it out as we go along.
-- Gary F.