I didn't find any way to prevent changing the desktop picture, but you could just change it back to the default every few seconds.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC -//Apple Computer//DTD PLIST
<string>tell application "Finder"
set f to POSIX file "/Library/Desktop Pictures/Galaxy.jpg"
if desktop picture is not file f then set desktop picture to f
Save the file above as
/Library/LaunchAgents/com.stackexchange.apple.65938.plist, change the owner, and load the plist:
sudo chown root /Library/LaunchAgents/com.stackexchange.apple.65938.plist
sudo launchctl load /Library/LaunchAgents/com.stackexchange.apple.65938.plist
The property list is loaded automatically when users log in.