Windows
1. Right-click on full image.
2. Select "Set as Wallpaper" or "Set as Backdrop."

Mac OS X
1. Hold down on the image.
2. Select "Save Image to the Desktop".
3. To set the new desktop, select "System Preferences" from the Apple Menu.
4. Go to "Desktop & Screen Saver".
5. Drag the image onto the pane showing your current Desktop.