find an image you like...
for example, go to google, click on their IMAGES option, and enter the name of, say, your favorite ski mountain. you will then have a page (if not pages) of images of that mountain.
you should be able to right-click on any image to get the option to save as desktop background. do that and you'll have that image as your desktop background.
obviously, the images you have available to choose from will depend on what you search for.