The question has been closed for the following reason "The question is answered, right answer was accepted" by Madison Tries Oct 07 '10 at 17:00
I believe you have to get a wallpaper that is the same resolution as both your monitors combined.
answered Sep 01 '10 at 19:20
Figure out the total resolution of your screens when combined (by your OS) into one single display and then go find or make desktop wallpapers that are at least that display resolution.
answered Sep 01 '10 at 20:13