The GIMP (GNU Image Manipulation Program) is a nifty little cross-platform tool. It's as good as Photoshop, but free (in both senses). A nice little feature of GIMP are the filters. Using the filters, you can turn some simple scribbles into aesthetically-pleasing desktop wallpaper. The "clone" feature is also really nice to remove unwanted parts of a picture.
The Way of the Ocean - example wallpaper done in GIMP: