Force a specific default lock screen image
This setting allows you to force a specific default lock screen image by entering the path (location) of the image file.
This setting lets you specify the default lock screen image shown when no user is signed in, and also sets the specified image as the default for all users
(it replaces the inbox default image).
To use this setting, type the fully qualified path and name of the file that stores the default lock screen image.
You can type a local path, such as C:\windows\web\screen\lockscreen.jpg or a UNC path, such as \\Server\Share\Corp.jpg.
This can be used in conjunction with the "Prevent changing lock screen image" setting to always force the specified lock screen image to be shown.
Note: This setting only applies to domain-joined machines, or unconditionally in Enterprise and Server SKUs.
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.
Force a specific background and accent color can be found in the Group Policy.
Please navigate to Computer Configuration - Policies - Administrative Templates - Control Panel - Personalization folder.