Preventing change to desktop background
I need to stop users being able to change the desktop background on their iMac's (OS 10.5).
Using WGM, I have disabled access to the "System Preferences" so they can't alter the picture and/or screensaver, however, it seems they are able to right-click in Safari and use an image as the background.
Is there any straightforward way for me to stop this from happening?
In addition, for those who have already set their own desktop background, is there a way to reset it to a picture of my choosing (one of the default pictures), and/or ensure it is reset on each restart?