You can change your desktop picture, or background, using the System Preferences application. Follow these steps:
1. Choose System Preferences from the Apple menu.
2. Choose Desktop from the View menu.
3. Choose a collection or folder from the Collection pop-up menu. Images contained in that collection or folder then appear in the selection pane below.
4. Click the picture you desire in the selection pane.
5. Quit System Preferences.
Note: The default setting for the Collection menu is Apple Background Images. The Pictures Folder menu selection refers to the Pictures folder in your Home directory. To use any other folder, use the Choose Folder menu selection. When you do, a navigation dialog appears as a sheet attached to the System Preferences window. Select the desired folder in the navigation dialog, then click Open.