The issue in this kind of games is generally that the game programmers have left glitches and shit like that unattended long enough for someone to realize that. The software the other guy was using could have been one of two things: a protection software designed for online games (i don’t know the name of any but i’ve heard a few exist) or a plugin, maybe a custom made one.

What kind of euristics can a program like that use for images is beyond me, My guess is it checks the user name, the avatar’s filename and allows manual blockage of what passes the automatic blocklist, so if the avatar isn’t called “anal sex.png” but show exactly that you can still block it manually.

Anyway your question should be asked on the specific forum for the game you’re playing, since there is no general rule for this kind of games when it comes to bugs, exploits and countermeasures.

@Thammuz thanks :)