If you ever want to change your login background on Windows, follow these steps.
Run – regedit
Navigate to: HKEY_Local_Machine\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
Change “OEMBackgrond” to “1”.
Now Run – %windir%\system32\oobe.
Create a folder called “info” and a subfolder called “backgrounds” if they don’t exist.
Copy any new wallpaper you want to that directory following these conditions:
a. Must be named “backgroundDefault”
b. Must be “.jpg” extension.
c. Must be less than 256 KB in size.
Download and run the tool at http://www.julien-manici.com/windows_7_logon_background_changer/