Click "Download" on the wallpaper you want, then:

Windows: Right-click over the image and choose "Set As Wallpaper."
Make sure under "Display Properties>Desktop" the "position" dropdown is set to "stretch"

Mac OS: Control-click on the link below and select "Download Image to Disk" Save the file to a location that you will remember. Then go to the Appearance Control Panel, and choose the "desktop" tab. Click on "Place Picture…" and choose the picture you just saved. Click on "set desktop."