Ah - good choice - this is one of my favourites! I suppose you what to know how to get it on the desktop now. There are more that one way to skin a cat when it comes to setting the wallpaper / bcakground as your desktop. You can either right click background wallpaper below and then click 'save target as...' (IE & Opera) or 'save link as...' (Firefox). Please remember where you save it to!Next goto where you set the desktop background for your particular computers operating system and select the image from the location where you saved it.
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'. When you're ready - you'll know what to do. I have faith in you. My guess is you already know. I'll be quiet then.
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