Careful with this one - can be slippery when wet. I suppose you what to know how to get it on the desktop now. Setting the background as your desktop wallpaper can be done in a number of ways. One way is to right click the wallpaper below and then select 'save target as...' (IE & Opera) or 'save link as...' (Firefox). If you do this - don't forget where you saved it! Now go to wherever it is you normally set your desktop backgrounds for your computer and selected the saved wallpaper.
A different way altogether is to right click the background below, from here choose 'save image as...' or 'set as Desktop Background' or 'Set as Background'. You'll have a flash of inspiration when the time comes. I think you have it in you. In fact you probably already know. I'll keep a lid on it now.
Return to wallpaper thumbnails.