Firstly ensure you have the correct wallpaper set as YOUR wallaper on your admin machine on which you are running Workgroup Manager (easier on a client, than the server).
Then in WM select your all users group and click on preferences along the top of WM. Then select details from below rather than overview. Overview shows the default icons for things you can manage, detials allows you to add specific plist files.
Click on + (add) below this window, then browse within your user profile to ~/Library/preferences/ and locate com.apple.desktop and select it. This will then basically deploy YOUR desktop wallpaper settings to your all users group the next time someone logs in.
Be sure that the wallpaper is located in the same place on the client machines as the admin machine you use to deploy the settings.
