If an 18 year old girl sends a spicy picture to her 17 year old boyfriend
If the 17-year-old sends one back, that's already a crime in many places. It could get him prosecuted for production and her for possession. If he uploads the picture to a chat group, the real fun starts.
The article says this is about nudity, not selfies. IDK how kids use instagram, but I'd guess that the senders are mainly kids sharing horny pics from somewhere else.