This tiny utility, aptly called Windows 10 Login Background Changer, can do it for you.
The utility is available from Github.
Changing the background image, however, should work just fine.
Windows 10 Login Background Changer| Github viaHow-To Geek
This Utility Can Change Your Windows 10 Login Image
Windows 10: The new version of Windows curiously doesn’t allow you to change the image that displays behind on the login screen when you first start y
