To make this your desktop click on the desired resolution, if you don't know what your resolution is it is recommended you choose 1024x768.

If you are using a windows system then once the image has loaded right click and select "Set as Wallpaper".

If you are using a macintosh system hold down the mouse button till you see the menu appear, then select "save image as". Open the control panel and go to "Desktop Pictures", where you need to first select "remove picture" and then "select picture". At this point select the image you previously saved to your hard drive.