There are a number of backdrops which can be used as the Wallpaper of your Windows Desktop. They come in different sizes so download the most appropriate for your system.
The pictures come a straightforward jpeg format (.jpg) and the basic instructions to download the pictures are as follows:
Click on the relevant link for the image size that you wish to download - this will open in a new window.
Save this file by right clicking the image and goto "Save Picture As..." (or similar).
Convert the downloaded image into a bitmap using any popular paint program.
Place this newly-saved bitmap image in your relevant windows directory (along with any other bitmaps which may be present there!).
Goto 'Display Properties' and choose the relevant backdrop.
Click "OK" or "Apply" and your wallpaper should update with your new chosen backdrop!
OR
Click the image size you wish to download - this will open in a new window.
Right click the image and you should get an option to 'Save As Wallpaper' (or similar).
Click This and the image will become your windows backdrop instantly.