Uh, have you tried using some basic math?
(# of Homescreens) x 1280 by 720...
You won't find anything in such resolutions...Just use a higher res picture and crop it. The Gallery crop doesn't work well, so I found some app called "Wallpaper Wizardii" which is awful looking, but works well for stretching stuff. I just use the "Stretch Landscape" option, it edits and saves a copy.