For MAC:
Click and hold down the mouse button on the thumbnail of the wallpaper you want to download. Choose "Download link to disk" from the menu that pops up. Open the "Appearance" Control Panel, under the apple menu items. Click on the tab marked "Desktop" and choose "Place Picture". Find your image in the window that pops up and double click it. Finally click on "Set Desktop". That wasn't too hard was it?

For PC:
Click on the thumbnail of the wallpaper you want to downlaod. Right click on the image that appears in the new window and choose 'set as wallpaper' from the pull down menu. Now sit back and enjoy the D on your desktop!