A browser default can be set for Google to have SafeSearch turned on (I believe you can do this within IE for all users via a Group Policy Object), but that is easy to get around for a moderately savvy student.
To date, the only answer I know of is to use brute force and that is to block all access to Google images. A Web-filtering rule can be set on the network's gateway or Web proxy to prevent access to that site. Of course, that totally turns off Google image searches and all of the students suffer. That's also not really an answer because there are quite a few search engines that will pull images from Google search and preventing access to all of them isn't an option.
So, in short, there is no easy answer, other than monitoring the students and ensuring discipline is handed down strongly and swiftly when students use the Internet in ways they shouldn't.
This was first published in February 2008