WIN PC:
Click on appropriate resolution. After image opens in new window right-click on it with your mouse, then choose "Set as Background" from the pop-up menu.
MAC:
Click on appropriate resolution. When image opens control-click it (or right-click with mouse) and select "Download image To Disk or desktop". Then go to the desktop, control-click the background and sleect "Change Desktop Background...". Browse to find the wallpaper file (usually on the desktop) and select it.