BuddyPress.org

r-a-y’s right, but I can’t help but feel that what you’re proposing could potentially be trouble with regards to XSS and people trying to break your site.

Another approach, with different benefits and drawbacks, would be to utilise either WordPress 2.9’s custom post types or just upload a number of background images using the Media Uploader; you’d still use the xProfile field to get users to select which image they want, but you write some code that switches the image selected with the actual image URL from the media library.