You can change Windows 10 login screen background using a third-party free app, Windows 10 insider build natively allows you to remove background picture on the login screen, for that, you need to open Settings app, click on Personalization > Lock Screen and toggle ‘Show Windows background picture on the sign-in screen’ button to ‘Off’.
This option doesn’t exist in Windows 10 RTM build 10240 (Microsoft might add it with future updates), but still you can able to get rid of Windows login screen background image though a registry tweak, here is how.
Removing default login screen background image in Windows 10 RTM build 10240
1. Open registry editor by typing regedit command in the Run dialog box
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System
3. Right click on System > New > DWORD (32-bit) value, assign name as ‘DisableLogonBackgroundImage‘ and set its value as ‘1’
4. Click ‘OK’ and exit the registry editor, lock the screen to see the change.