First, because we were having trouble with spammers adding bad things to their profiles, you have to make your first 5 posts before you can edit your profile (probably why you're having trouble finding it right now).
Once you get to 5 posts, click on the “PROFILE” link on the menu at the top of any page on the forums. Then, there will be a menu on the left. The second section has the heading “Modify Profile”. Within there, there's a link called “Forum Profile Information”. Click that link and you can set a picture, signature, and various other things.