OK, that explains the personal "liberty" portion. \:\) The other side of that is just performance. I'm just not sure how often the OS will try to go out and "touch" the file being used for the background image. I would think the most expedient method would be:

1. Configure each system to use a consistent name/location as the background image.
2. In your login script, check the difference between the file on the share and the one on the local computer. If they are different, copy the share to the local system.

Then all you have to do on the server is change the contents of the file to what you want. If you don't want anything displayed, just put an empty file there with the correct name.