Windows 10 lock screen background different on startup

This solution works for Windows 10 Enterprise.

Go to the following Registry entry (or add it):

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Personalization

Add a new String key with the name LockScreenImage and the value of the complete file path of the image.

Alternatively, you can edit the group policy:

Computer Configuration > Administrative Templates > Control Panel > Personalization > Force a specific default lock screen image

I found this solution here.

I'm not sure if this works on non-Enterprise installations, or if non-Enterprise installations even have this issue in the first place. Let me know if this doesn't work for you.