How to change your background:
1. Click the System Preferences icon in the Dock. (or Apple menu and System Preferences)
2. Click Desktop & Screen Saver, (and select Desktop).
3. To select the kind of desktop image you want to use, you can use an image that come with your Mac or use your own picture.
4. To use your own picture:
Select the Picture folder under “Folders,” if the picture you want is Stored in your Pictures folder.
If your image is in another folder, click the Add button (+), then find and select the folder that is in your picture. Then, click Choose.
5. Select the picture you want in the box on the right.
Video Turtorial
Mac101 