mikechaik
gpedit.msc
system USER
Administrative Templates\Desktop\Active Desktop
Active Desktop Wallpaper
At least Microsoft Windows 2000
Specifies the desktop background (wallpaper) displayed on all users' desktops. This setting lets you specify the wallpaper on users' desktops and prevents users from changing the image or its presentation. The wallpaper you specify can be stored in a bitmap (*.bmp), JPEG (*.jpg), or HTML (*.htm, *.html) file. To use this setting, type the fully qualified path and name of the file that stores the wallpaper image. You can type a local path, such as C:\Windows\web\wallpaper\home.jpg or a UNC path, such as \\Server\Share\Corp.jpg. If the specified file is not available when the user logs on, no wallpaper is displayed. Users cannot specify alternative wallpaper. You can also use this setting to specify that the wallpaper image be centered, tiled, or stretched. Users cannot change this specification. If you disable this setting or do not configure it, no wallpaper is displayed. However, users can select the wallpaper of their choice. Also, see the Allow only bitmapped wallpaper in the same location, and the Prevent changing wallpaper setting in User Configuration\Administrative Templates\Control Panel. Note: You need to enable the Active Desktop to use this setting. Note: This setting does not apply to Terminal Server sessions.
Код:

HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System!Wallpaper
!parameter означает: parameter=dword:00000001 (запрещено) или dword:00000000 (разрешено)
Источник:
Group Policy Reference
Также, можно указать стиль обоев
Код:

HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System!WallpaperStyle
Поскольку настройка пользовательская, а не машинная, то надо импортировать ее в профиль Default User и либо удалить все созданные пользовательские профили, чтобы при последущем создании они такую настройку имели либо внести настройку в них.
Guide :: Customizing the Default User Profile