Just to be clear, running gpupdate /force on the server will only update group policy on that server, not the users' workstations.
The command will need to be run on the client machines in order to set it, but even then it won't change their wallpaper until they log off and log back on again. Group policy is refreshed on login anyway, so running while running gpupdate /force will force the policy to refresh, it won't change the wallpaper immediately.
Best thing to do - wait until they log off/on.
