As I knew in terms of web security solutions mostly based on :
1. Database of 3rd party vendor, which's provide world wide database for restricted website such as porn, gambling, social networking etc (i.e: commtouch)
2.Based on ReGex or phrases, this features mostly have in secure web gateway solutions which's the concept such as avast webshield blocking based on phrases that we stated in the list
3. Based on ip address of the web server (this method frequently used by administrator, since a lot of webserver has a web server more than one server and ip public)
And according to your information that web blocking could blocking based on the pictures of the website that is new for me. Because as I know this strategy more in depth used as antispam filtering which's for each incoming messages which's contains such as ** SPAM ** or other nude picture shall be blocked or rejected.