Firefox and Desktop Wallpaper

If you do a Google image search in FireFox or Safari, for example, and right click on the picture you can save as desktop background. I was able to block the kids from doing this in Safari, but have not been able to block if they use FireFox. Any ideas?

Blocking System Preferences does not block changing the desktop using a 3rd party program nor does it block changing the dock.

This is correct. I did try this and thought it was solved, but the kids still found a way. However, after doing a little more research, I did the following which did work.

Thanks to Tom Rice who posted this fix in another area.

Set up an account(ideally on a machine with workgroup manager installed) with a background that you want. Ideally this will be an image that is on all the machines you wish to manage. Using one of the predefined Apple images works. Log into workgroup manager and select the group or computer list you wish to manage. Click on the preferences for the group and then Details. Click on the plus sign to add a new preference manage. Navigate to the ~/Library/Preferences folder of the account you had set up previously and then select the com.apple.desktop.plist file. By default the settings for this file (such as desktop image) are set to be managed Once. Meaning that they will be set when the user logs in but can be changed after that. To change this click on the settings you want to have managed all the time. Then click copy. Highlight the word Always and then paste. When asked if you want to replace the MCX settings select yes. Apply and done. Users will then have the background you defined and will not be able to change it via any method. Of course locking the desktop preference is a good move too.

This worked and I can already hear the kids moaning about "big brother" taking away their fun.

Thank you everyone for your input on this issue.