Well what I did was find something I like. Then I upload it to Flickr (just what I use) then I think you copy the url in the section of this site for avatars and it should come up, maybe.

Best answer for you, contact Tom or maybe even JJ and ask them how to do it. JJ actually made my avatar (made the plunger to shoot). For my avatar, I just typed in avatars, highlighted images and started looking until I found the one I liked.