Do you want to put your own picture of choice in there or you just want to use one of the standard avatars?
Go to usercp and edit options.. scroll all the way down to where it says change avatar and click that. It should then give you a choice of avatars we have here on the forum, or select 'use custom avatar' to upload one from your computer. It has to be no bigger than 72X72 pixals and no larger than 7kbs file size.
I got to "change avatar" but I must have a setting selected so I can't view any avatars. All I see is white boxes with red x 's. And that is on every screen.