Thread:My Running Man/@comment-22439-20130715122421/@comment-1330470-20130715152508

You can upload an avatar from your userpage. See Help:Avatars.

If you're talking about an actual background image, this mostly has to be done via ThemeDesigner, see Help:Theme designer. But you have to be an admin of the wiki to do it, so you will not be able to do it here unless you edit your personal CSS.