I have two teenaged sons. Nude selfies proliferate, and I have to warn them that looking at, or downloading, pictures of their girlfriends or classmates could constitute a sex offense under certain pornography laws.
In the UK, you don't even have to look at it - if you receive a completely unsolicited sexual image, by email, text or other messaging service, and don't even open the messaging software, you are in possession of an indecent image.
In the wording of the law, if you open it you become a pornographer, and if you take your phone to the police to show them, you are guilty of sharing an indecent image and they take your phone as evidence; even if they decide they aren't going to prosecute you, they're going to prosecute somebody, and even if they decide not to do that, they can't give an indecent image back to you, and they aren't allowed to delete files from your phone, so they have to keep the phone.
That, in fact, has proven to be one of the biggest deterrents to teens sharing naked selfies over here - if just one person in a group chat posts an indecent image, everybody involved in the chat, even if they'd just left it running in the background, loses their phone as evidence, and they don't get the phones back, ever.