Change your desktop look and feel by applying a nice background called wallpaper. You can change the mac wallpaper to one of the patterns, colors, your own or pictures or wallpapers that come with Mac OS X.
If you want to apply or change desktop wallpaper in Mac OS X, then follow the steps to do so.
You can also see How to change screensaver in Mac OS X
Click on Apple logo and go to System Preferences
In Personal click on Desktop & Screen Saver option
Select Desktop button. In left panel of screen you’ll find all wallpapers which you have on your system
Click on any wallpaper of your choice, immediately it became applicable on your screen
You can also set timer to change a wallpaper randomly after a fix time. So mark Change picture check box and then set time from a drop menu according to your choice
We set here Change picture time after every 30 minutes, Random order and Translucent Menu Bar check boxes are also marked, and just quit from the screen to apply all the changes made