Facebook Timeline Cover
To download the image, right click (CTRL + click on a mac) on the image and select "Save Picture As..." Select the destination folder on your local computer where you would like to save the image. Click "Change Cover" at the bottom-right of your Facebook Timeline Cover image and follow the prompts.
PC USERS: Select size then right click on the image and select 'Set as Wallpaper'.
MAC USERS: Select size and then CTRL + click on the image to save to your hard drive. Open your System Preferences and click on the desktop icon. Drag the image you want to place as your desktop into the 'Current Desktop Picture' thumbnail.