Windows 7: Customize Logon Screen Wallpaper
  1. Start → Run… → regedit.exe

  2. Open HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background

  3. Switch the key OEMBackground to value 1 (create the key if it doesn’t exist, it’s a DWORD).

  4. Navigate to the folder C:\Windows\System32\Oobe

  5. Create a new sub-folder called info.

  6. Go to info and create a new sub-folder named backgrounds.

  7. Put the custom wallpaper there; it must be titled backgroundDefault.jpg, it has to be a JPEG file, must not exceed 256 KB in size and should match the dimensions of the primary screen (otherwise it will be streched to fit the screen).