Click the System Preferences application icon in your Dock. If you do not have the System Preferences icon in your dock, access System Preferences from the “Apple” menu on the system toolbar.
Click the “Desktop & Screen Saver” pane from the System Preferences menu. Choose a folder from “Apple.” If you do not see folders, click the small gray arrow next to “Apple” under the wallpaper list. Macs come with wallpapers in the categories of Nature, Plants, Black & White, Abstract and Solid Colors. Click on a folder to display its wallpapers.
Select a format from the drop-down menu above the wallpaper icons. Choose between “Fit to Screen,” “Fill Screen,” “Stretch to Fill Screen,” “Center” and “Tile.” Check the “Change picture” box under the wallpaper icons to enable wallpaper slide show mode. Set the rotation rate for every five seconds, minute, five minutes, 15 minutes, 30 minutes, hour or day.
Click “iPhoto Albums” to select a photo from your iPhoto library to set as your wall paper. If you do not see a list of your iPhoto albums, click the small gray arrow next to “iPhoto Albums.” Click any photo to set it as your desktop.
Click the “+” sign under the wallpaper list to add your own folder of wallpaper images. Click an image within the folder to set it as your wallpaper.
Use any image as your desktop wallpaper by dragging and dropping it into the wallpaper box in the Desktop & Screensaver tab of System Preferences