How to Prevent Changing a Desktop Background in OS X 10.6
The background image on your OS X computer is controlled from the System Preferences menu. When you access the System Preferences menu, you can change the background image and the length of time that you want it to be displayed before the screensaver comes on. OS X gives users the ability to lock these preferences with little hassle. This process only takes a couple of minutes to perform and doesn't require advanced knowledge of the subject.
Click the "Apple" logo at the top of your computer screen.
Click on the "System Preferences" menu.
Click the "Security" tab.
Click the "General" pane.
Click on the "Require password to unlock each System Preference pane" option.
Exit the window.