Filtering Google images with Sophos Endpoint Protection and Web Appliance WS5000

I am looking for advice on filtering out image results on the Google images page.   We are currently running the Endpoint protection client on all of our systems and we have the Web Control policy configured to use the Sophos Web Appliance.   Here is the problem.  When a user performs an image search on google.  Lets say, "nude women".  The browser fills up with photos of nude women.  What can I do to block this?  I realize that Google indexes the web and retains the images it finds on it's own servers.  Any ideas or advice would be greatly appreciated.   We do have the word search feature turned on in the Web Appliance (WS5000).  So, if someone does search for "nude" or "naked", the search will end up on a report.  However it isn't blocked.  

Thanks!