PC Instructions:

Click on the size wallpaper you wish to download. A window will open up with your wallpaper in it. Right-click on the image and select "Set as Background" or "Set as Wallpaper"

Mac Instructions:

Click on the size wallpaper you wish to download. A window will open up with your wallpaper in it. Right-click on the image and select "Save Target As...". Save the wallpaper to a folder on your computer. Click on the Apple menu, then select 'Control Panels > Desktop Pictures'. Find where you saved your wallpaper, then select open followed by 'Set Desktop'.