The sexy picture in the monitor is 640 x 480in dimension.  If this matches your monitor setting, proceed to step 2.  Otherwise, maximize the browser window (enlarge it so it fits your entire screen) then click the appropriate number on the right to get the correct image.

Right-click the picture in the monitor on the left.

Choose "Set as Wallpaper" from the right-click menu, as illustrated below.

Your boring old desktop now has Nasty Wallpaper.

Netscape users:  If you get only a small image as your wallpaper.   FIRST double-click the image in the monitor to see it full size, then right-click the full size picture and set it as your wallpaper.