There is a bunch of ways to set an image as your Desktop picture, such as using Quicksilver or using Automator.

Recently, I've found another quick way to do that. Do you want to know what's that?

Set Desktop Picture with Safari

1. Open Safari
2. Drop Image File to the Browser Window
3. click on Image
4. Choose Use Image as Desktop Picture
   And you will get your Desktop background changed in an instant

Actually, you can do this trick on all web browsers. But I love Safari, so I mentioned Safari first *evil glare*.

Set Desktop Picture with Firefox