Locate the following folder C:\Windows\System32\oobe\Info\Backgrounds (“C” is your Windows Seven Installation drive)
Copy your favorite image file to this folder and rename it as backgroundDefault. Note that the image must be in JPG format and the size should be less than 256 KB.

You are done. Reboot your system and see the change.

The following files (sorted by width-to-height ratio) are supported in C:\Windows\System32\oobe\Info\Backgrounds folder:
  • backgroundDefault.jpg
  • background768×1280.jpg (0.6)
  • background900×1440.jpg (0.625)
  • background960×1280.jpg (0.75)
  • background1024×1280.jpg (0.8)
  • background1280×1024.jpg (1.25)
  • background1024×768.jpg (1.33-)
  • background1280×960.jpg (1.33-)
  • background1600×1200.jpg (1.33-)
  • background1440×900.jpg (1.6)
  • background1920×1200.jpg (1.6)
  • background1280×768.jpg (1.66-)
  • background1360×768.jpg (1.770833-)