Including, I believe, a rather massive body of drawings and animation films from Japan (I'm thinking of Hentai involving underage schoolgirls).
One does not simply forbids those "to protect the children" (no one under 18 is actually involved in the making of such… art), so there must be some harder to justify reason (like good old puritanism, or priming effects).
This is not illegal at the country-level in the US, however such laws do, hideously in my opinion, exist in England and Wales, Northern Ireland, Ireland, Australia, New Zealand, Canada among other places. They also exist as "oscene" at the state level and thus you may be liable depending on state law.
There was recently a case in Austrila in which a prisoner was brought before a court on the charge of producing child pornography, while in prison -- how? He wrote short fiction stories and they were found by a prison guard. Similarly longer ago a case in Australia decided that children in the Simpsons do indeed qualify as children, beacuse although they do not share many bodily features of real humans, the genitals as depicted in the pornography did share characteristics to those of real humans, which was apparently enough to secure a conviction under the charge of possession of child pornography.
Canada similarly has illegalised possession, even of story material depicting sexual acts involving a minor. To me at least it is ridiculous to think that one may be libable under the law for prison time of up to three years for possession of a drawing.
Devils'a advocate: But some information is illegal in USA, like kiddie pron, terrorist stuff, stolen trade secrets etc...so we have to make sure it doesn't enter USA.
On the other hand, they can send it via the internet, making border searches less effective. And since they're really burdensome and affect privacy of everyone the court might do away with them. Hopefully