Customize the Windows Log-On and Log-Off Screen
You can customize the log-on screen (shown when selecting an account to log into) and log-off screen (shown when shutting down) of Windows with your own background image. Keep in mind, the JPG image must be less than 256KB in size and preferably match the aspect ratio of your screen as it'll be stretched to fit.
Modify or create the following in the Windows Registry:
DWORD Name: OEMBackground
DWORD Value: 1
Next, copy the custom image into the %windir%\system32\oobe\info\backgrounds folder. Create the info\backgrounds folders if they don't exist. Rename the image to backgroundDefault.jpg, reboot, and you should now have a custom log-on image.