Forcing windows 7 theme to change across the whole network

Hi,

I am working for an advertising company as the IT apprentice ive been tasking with changing the background off all the machines on our network.

This would usually be an easy task as i can just set a GPO to change the screen saver, the issue I have is they want to the background to rotate between 5 images and to my knowledge you can only change windows 7 theme through GPO to new users.

Is there a way I can push a script out over the network that changes the windows 7 theme or is there any group policy i can change??

Any help is greatly appreciated.

Thanks

Jack