Once you've decided which wallpaper you'd like to use (see below), you'll need to click on the relevant screen resolution size (see below if you're not sure what yours is) just underneath the image.

Having clicked on the required resolution size above, a new window of the image should open. To get this image to display on your computer's desktop, all you need to do is right click on this image (the one in the new window) and select Set as Background from the mouse menu. Et voila!

Note:
If you are uncertain which screen resolution size you need, you can find this information in your Control Panel. Within here, select Display and look under the Settings tab.