To change the login wallpaper on Windows 8 you must do some little steps. First of all replace the default login wallpaper with your wallpaper. Your wallpaper should have following name: img100.png
Place it on following location and replace the existing file: C:\Windows\Web\Screen\
Please note: You must take ownership first, before you can overwrite the default wallpaper.
After that you must delete following directory: C:\ProgramData\Microsoft\Windows\SystemData\S…
Please note: You must take ownership first, before you can delete the directory.
Now you must give the System user Full Control rights to the SystemData directory. After that you can restart your computer and you should have the new login background. 🙂