Some people become sexually active earlier than others. Trying to stop her is just going to cause her to dislike you and hide her life from you. If I'm understanding correctly, they met organically at an after school program. From the sound of it, they probably have one of the most normal and healthy relationships you could hope for these days. It's not like he's a stranger from the internet preying on young girls. The difference between a 13 year old and a 17 year old is big, but it's not like he was cruising the internet for her. Note, he is ALSO A KID. You may have lost your perspective, but 17 year olds aren't that much farther along than 13 year olds are. What makes you think that you'll stop feeling the same way you do about your daughter's safety once she has turned 17? You're treating him as this scary adult preying on kids, but he's probably just a nice boy who happens to be in the same after school program as your daughter.
She's lying to you because you're making her lie to you. You told her from the start she couldn't date this guy when she approached you and asked you reasonably about it. Of course she's going behind your back, but it's your fault, not the boy's and not hers. She's sending nude pics and explicit texts because that's the only way she can communicate with this guy who she's being prevented from seeing in person. You cut off her options for a healthy relationship that she can share with you, and are acting surprised that she's not sharing it with you.
Have you considered starting over? Instead of calling this boy's parents to tell on him or have them restrict his actions, why not get together with them and this boy and your daughter for brunch? Get to know this boy and his family, he might be a decent guy.
The child pornography concerns are really serious, but it doesn't have to be something that they are getting in "trouble" for. There are serious legal concerns, you don't want them hiding these things from you to save themselves from some ambiguous trouble when there are worse things that could happen.