make an album and put some photos in it (like the one I have) then go to User CP, and at the top left collum there should be somthing about editing the profile. Pick a photo, copy the url from the specific photo's page (not the one with several photos on it. Just the one) and paste it into a box there that says Background Photo URL. You can alter the color of text and stuff there too.
I'll post a screenshot if you need it.