The version appears on this page is the full size image,
so all you have to do is:
1. Right-Click on the desired image.
2. Select "Set as Background".

If your screen resolution is above 600x800, you probably want
to stretch it so it will cover the whole desktop.
To do so, do the following after you set the image as a background:
1. Right-Click on the Desktop.
2. Select Properties.
3. Click the Background Tab.
4. In "Display" Select "Stretch".
5. Press "Apply" and "OK".