There are default avatars provided with the software and you can pick one of those by going Profile >> Forum Profile (which is in the drop menu under Profile) >> Personalized Picture. Click Save at the bottom of the page after selecting the avatar you want.
If you want to use another image you can have anything up to 150 pixels high by 125 pixels wide. To make things easier for the admins we don't allow everyone to upload directly to our server. You'll need to have a Photobucket or similar account to host (and link to) your own images online. They're free and easy to set up, and they allow you to have quite a sizable online album.
http://photobucket.comAlternatively if the image you'd like to use is already online give me a link to it and I'll turn it into an avatar for you.
Devious, treacherous, murderous, neanderthal, sub-human of the West.