For Windows Users:
With your mouse pointer, right click on the image and choose Set As Wallpaper from the pop-up menu that appears in the browser window.

For Mac Users:
Click the image while holding down the command (Apple) key on your keyboard and save it to your computer. From the Apple menu, open the Control Panel and select Appearance. In Appearance, select the Desktop tab. Drag the saved image onto the preview window in Appearance, then click Set Desktop