Here's RFF's collection of wallpaper. Just choose the wallpaper that is the resolution of your monitor, and right click on "Set as Wallpaper". If you'd like to contribute wallpaper to RFF, see the contribute link below.
Submitting Wallpapers
I always appreciate any donations and submissions to the site! Make sure that any wallpapers that you submit to RFF are at least 1024x768 resolution or higher and that they are your own creation.