- Run regedit.
- In Registry Editor, go to following key: HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System
- In right-side pane, create new DWORD DisableLogonBackgroundImage and set its value to 1.
- Restart your computer and it'll remove the background image from login screen.
If you want to restore Login Screen background image in future, delete the DWORD DisableLogonBackgroundImage.
From How To Change Sign-in/Login Screen Background Picture In Windows 10
Pictures are located in C:\Windows\Web\Screen
One other setting: Settings -> Ease of Access -> Other Options -> Show Windows Background -> Off