Prevent changing lock screen image
Prevents users from changing the background image shown when the machine is locked.
By default, users can change the background image shown when the machine is locked.
If you enable this setting, the user will not be able to change their lock screen image, and they will instead see the default image.
At least Windows Server 2012, Windows 8 or Windows RT
Not Configured - No registry values are specified.
Disabled - Any values that would be created by the policy in the registry are deleted.
Enabled - Adding, modifying or delete values in the registry.
Prevent changing lock screen image can be found in the Group Policy.
Please navigate to Computer Configuration - Policies - Administrative Templates - Control Panel - Personalization folder.