Best way to do them is just click the thumbnails below, then right click 'save target as' and save it somewhere on your computer. Right click your desktop and select properties, then follow the instructions for setting the picture as your desktop. If you are not using a 1268 x 760 resolution, you should also select 'stretch' to fit the screen
You can also right click the large images and select 'set as background', but i prefer the above.