First you should know the default Windows 7 wallpaper images are located at C:\Windows\Web\Wallpaper.
How to Change the Desktop Background in Windows 7:
1. On a empty space you right click on the desktop and select Personalize on the menu.
2. At the bottom of the new window, Click on the Desktop Background link.
3. Select a Picture Location --> Select a Desktop Background --> Select the Picture Position
4. Click OK button to apply. Then it will be done.