There is a very popular scam which has come to be known as "sextortion," which is a shorthand for sexual extortion and this sounds like what you've experienced. Unscrupulous people hover around on chat rooms and dating sites, and they lure in their victims by assuring them that they are of age but then engaging them in sexual chat and pictures. Then, all of a a sudden, you learn that she is still a minor. Sometimes, as in your case, she tells you that herself. Sometimes, you don't get the news until hours later or the next day when you get a phone call from the alleged parent. And what the "parent" wants is money.
That's blackmail. You need to block all contact with the so-called minor and her "father: and send no money . You should never give in to an extortion request. If you pay them, you'll have to keep paying them for the rest of your life. They only have power over them if you give in to them.
When she sprung on you the fact that she was underaged, you should have immediately left the conversation and blocked her number, which would show that you were never intending to pursue an underaged woman. Instead, you didn't do that. You continued to chat with her for days afterwards, you've got a photo of a possibly underaged girl on your computer, which is a serious felony charge, and you sent an explicit photo of yourself as well.
You're lucky that this is going to be a scam and that there probably really isn't any minor at all, because you can't go to the police and report the blackmail without admitting to crimes that could get you charged with felonies and require lifetime registration as a sex offender.
I don't think you're going to get in trouble, however, because if she were really underaged, she'd be guilty of a crime by sexting you and her father would by blackmailing you. So chances are very high, there's no real father here, no minor at all, and nobody who's going to dare go to the police. Again, block all contact from here and cancel your membership on the dating website, and call their bluffs. The odds are high that nothing at all will happen except that they will move on and try to set someone else up for extortion.
I cannot guarantee you that you have nothing to be concerned about, however. So in the event that you hear from someone who claims to be a police officer, or if the "minor" and her "father" try to reach you another way, say NOTHING about this incident to the police except that you do not wish to discuss anything until you can reach your lawyer, and then go out and retain counsel.