Hi Chris,
This looks to be the only way to specify wallpaper in GP.
User Config\Administrative Templates\Desktop\Active Desktop\Active Desktop
Wallpaper
"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."
br,
Denis