What is the best resolution to offer images for download for desktop wallpaper?

You should include version for all these sizes:

640 x 480

800 x 600

1024 x 768

1152 x 864

See Digital Blasphemy for and example:

http://www.digitalblasphemy.com/

Images scale when used as wallpaper. You only need to respect the 4:3 display aspect ratio so 800x600, 1024x768 .. 1600x1200 all good.

use multiple sizes, the most common resoulution being:

800x600

1024x768

1600x1200

and better if you can provide image in it's maximum size (not yet resized), so people that has out-of-common resolution would be able to resize it themselves.

These sizes are common since

a) 800x600 is set as default resolution on XP

b) most LCD monitor has native resolution of 1024x1768

c) most high enders might prefer higher resolution, most commonly is on 1600 x 1200

1024 * 768

bcz low density and high quality. ;)

1600x1200, then most anyone can use them