[jasode]

>But in your example it seems reasonable to me to just block the post where it is illegal.

Maybe I'm misunderstanding what you mean by "just block the post" but it seems the algorithm for writing:

  is_illegal(image, country)==true

.... is currently not possible to write with accuracy. (Relevant XKCD: https://xkcd.com/1425/)

If it's a large-scale service used by a billion people posting user-generated content, you have to rely on algorithms to filter it instead of depending on users manually flagging posts. Users themselves don't know their own country's laws and would flag things like the Vietnam naked girl photo as "illegal" when it really wasn't.

Even if a machine learning algorithm can be trained to flag a specific cartoon of 3 religious figures having sex as illegal, there are near-infinite variations of other drawings showing the same thing that would get passed as "legal". In fact, it doesn't even have to be a totally different comic by a different artist... just changing 1 pixel might do it: https://news.ycombinator.com/item?id=15577885