Each year when we update computers I receive questions about changing desktop backgrounds. Here it is! If you want to have a specific picture as your background, you must have the JPG file available - higher resolution is better, as pictures get fuzzier when they are enlarged.
Apple Menu --> System Preferences --> Desktop & Screen Saver
You can choose from Apple's backgrounds or click the "+" underneath the list to add your own photo to the list, or choose from your iPhoto library.
Right-click on the desktop and choose "Properties," then click the "Desktop" tab. Choose from the list or click "Browse" to find your own saved photo.