Well...
800x480??? That picture says1200x720, so which is right??? If that's right, then that could be why, as a guess... Shrink it down, using the menu in that picture, or else increase the background picture size to compensate, if that bigger number is the correct one... if you understand what I mean??? Hope this helps, but without more information I can only guess... Also depends on if you're trying to leave it at the imported size or change it, if I'm not mistaken???