If you are looking for a dual monitor wallpaper (2560×1024) in one of the following categories then dualmonitorbackgrounds.com is the site to see.

Abstract

Animal

Astronomy

Celebrities

Computers

Crafted Nature

Gaming

Industrial

Macabre

Microscopic

Mobile Technology

Music

Nature

Popular Culture

Science Fiction

Sports

Vehicles

The European Homepage for The NASA/ESA Hubble Space Telescope hosts an impressive array of awesome (truly awe inspiring) images and wallpapers of our universe.
The images are formated in 1024×768, 1280×1024, 1600×1200.
My favorite part is the High Def videos that are available on the site. They have 720P and 1080P videos for download.We are very excited [...]

Desktop Pictures hosts a lot of great images in the following categories.

Scenic
Florida Beaches
Abstract
Clouds
Flowers
Colorful Leaves
Haiku Letters
Black and White
Expressive Leaves
Holidays

Many are available in 1152 x 768, 1152 x 864, 1152 x 870, 1280 x 800, 1280 x 854, [...]

wallpapers (1024×768 minimum) is the mother load of high res flickr groups. It is chock full of images tagged with wallpaper at a minimum resolution of 1024×768. 36,669 images and counting. This will take a while to go through.

Professional Wallpapers hosts a few HD images in a number of different formats. including 2560×1600, dual monitor, HDTV and PSP/iPod formats. Nice work.

desktopography is an exibition, a showcase of nature themed desktop wallpapers created by designers worldwide.
This site is awesome! great visuals and great music! Lots of great, hi def and large format wall paper and all around great design. Check them out.