2. You can either choose from the pre-loaded images by selecting folders on the left or click on the '+' button near the bottom left to select your custom image. Just navigate to the location where you saved the wallpaper and click choose, this will set that image as your desktop background.
3. If you have other more background images in the same folder and want them to automatically switch, select 'Change picture' just below the wallpaper thumbnail preview area and then choose a specific time period, ranging from 'every 5 seconds' to 'every day' or you can even set the wallpaper to change everytime the mac wakes from the sleep or whenever you log in.
5. You can also change how the wallpaper fits the screen. Just use the menu above the wallpaper thumbnail preview area and select an option such a 'Fill Screen', 'Center', 'Tile' etc.