This incantation might help you dig some cc by and by-sa files from flickr.
http://flickrhivemind.net/flickr_hvmnd.cgi?search_domain=Text&textinput=wallpaper+desktop+background+-seamless+-tile&search_type=Search+Text&photo_number=100&photo_type=250&sort=Interestingness&page=1&tag_mode=all&license=4&license=5
