Photography of mutilation (which is gore) itself actually isn't illegal, only the act in the mutilation is. Spreading photography of mutilation does not fall under obscenity laws as being illegal.
Also, "anonymous" doesn't mean shit, because the way the Internet works is that you literally give a website your IP address simply by connecting to it. 4chan's site moderation has collaborated with the authorities in many ways whenever a substantial amount of illegal content has been posted on there by giving the authorities the poster's IP address.