Under Internet Policies, the administrator can activate the Search Engine Policy > Safe Search option. This option activates the safe search feature available in major search engines.
The safe search feature generally works using color shades and adult-related terms, thus blocking all images or searches that match the parameters already set by the search engine. Parameters include several flesh tones and obvious terms.
NOTE: Google is redirecting all HTTP requests to HTTPS. So, in version 2013 for google safe search, HTTPS inspection must be enabled for this feature to continue to work properly. In the version 2015 you have the option to select: "Google Https Redirect". With this option checked, the traffic will be redirect to HTTP and it will work even with HTTPS inspection disabled.