from what I recall you are able to set it from the registry. create this key with a string value called 'LockScreenImage', this string should point directly to

from what I recall you are able to set it from the registry. create this key with a string value called 'LockScreenImage', this string should point directly to an image, i.e: C:\.... HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Personalization throwaway because of not wanting to get arrested for hacking the registry