Then from a picture, tell me what the exact age of the nude person is.

Again, we need to know the exact year and day. 17y364d is illegal but 18y is legal.

Not sure how this answers my comment above.

It points out the failing of comparing an action (killing someone without cause), and passively receiving a picture.

That picture does NOT have enough information to determine if its legal or not. And even as simple as 1 day can be the difference between legal and not.

And of course, is also thethe hypocrisy of sex being legal at 16, but pictures are 'child porn', when they are demonstrably NOT children.

Well, there are MOST DEFINITELY images that are, UNAMBIGUOUSLY, VERY illegal. It is not hypocritical to say that detecting such images one a device means that someone should look into it, because something got VERY WRONG there.

So yes, MANY pictures already exist that do have more than enough information to determine that they are VERY ILLEGAL.

If you can't apprehend that, I don't know what to tell you.