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To set the image as wallpaper, click it to open it in a new window, right-click on it then choose Set as Background (or Wallpaper) from the pop-up menu or choose "Save Picture As" to save the picture.
Once a file has been saved to your computer, Click on Start, Settings, and Control Panel. From there, double click the "Display" tool. Then select the "Desktop" tab at the top of the box, from there you can browse for the Image you want.
For Windows 7, click on Start, Control Panel, Appearance and Personalization, Personalization, and then Desktop Background. From there you can browse for the Image you want.