To the very best of my knowledge, there is no law that states that two 15-year-olds cannot send pictures of their naughty bits to each other. It might be stupid (and it is) but not illegal. Learn a lesson from this. It just takes one angry person, and the whole world is looking at your junk. Manipulated by the wrong person, it COULD wreck your life. And I don't mean, I'm 15 and it's "over" wrecked. I mean like REAL blackmail that can really wreck your adult life.
I understand her dad freaking. I just don't agree with the way he's handling it. If I was in your shoes, I would 1)Tell him that you have already told your parents and it is being dealt with on their end, and 2) it is NOT illegal, so drop the crap. 3)Your relationship with her is now fried, so you might just as well tell her what's up (as opposed to sending a pic of what's up LOL) Why? I can get myself into trouble JUST FINE. But it sounds like she was just as much of a participant as you. I DO NOT like taking heat that ISN'T all mine. Credit where credit is due, spit where spit is due.
I truly loved (or so I thought) my high-school sweetheart, Amy. We dated for three years, and really did start to think we'd marry. My wife's name is Melanie. You do the math.
Now- since you've got all this pent up energy, go build something! THAT, you can take pics of!
EDIT- Dang, I wish I only had 15-year-old problems!