FIRST, you gotta be a forum supporter. (I have a feeling you already know the rest, but) Once you've got that settled, you go to "User CP" then "Edit Avatar" under "Settings and Options". From there, you'll see what to do.
I just used 'Paint'. Open your picture in Paint. Make sure your picture is a perfect square by using the 'Select' tool to...... to be continued, but maybe you will figure it out from there! if not, just PM me and send me the pic you want to use and i'll fit it down to size for you when i get home in a few hours.