Just the facts, Ma’am!
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background]
“OEMBackground”=dword:00000001
C:\Windows\System32\oobe\Info\backgrounds\backgroundDefault.jpg
That’s it! No 400 word essay necessary to explain this.
To help with the Registry entry – zipped .reg file to enable login background image.