Go to your user cp button at the top right of the forum screen.
Click on "Edit Options"
The avatar stuff is at the bottom.
I found a website that has a bunch of avatars and just linked to that. You need to have the avatar at 50x50. I'm not sure how to link an avatar from your computer. The forum server stores the avatar once you link it though. Sorry I couldn't help you more. I would like to learn more about how to do this stuff!