Force a specific Start background
Forces the Start screen to use one of the available backgrounds, 1 through 20, and prevents the user from changing it.
If this setting is set to zero or not configured, then Start uses the default background, and users can change it.
If this setting is set to a nonzero value, then Start uses the specified background, and users cannot change it. If the specified background is not supported, the
default background is used.
At least Windows Server 2012 R2, Windows 8.1 or Windows RT 8.1
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 Start background can be found in the Group Policy.
Please navigate to Computer Configuration - Policies - Administrative Templates - Control Panel - Personalization folder.