Reciprocal
I'm pretty sure it's also an offence for her to have initially shared the image with you as she was under 18 at the time and I don't think there is a distinction in law between a person sending a (sexual) image of themselves or of anyone else under 18, your just not allowed to do it. I would tell the police or your lawyer that she distributed it to you and push for her to be prosecuted with distribution of child porn. It might not help you but will potentially get the girl also facing being put on the sex offenders list and she will be forced to tell the truth to get her own self out of this, which might just help your case or at least give her a taste of her own medicine. I would hope this wouldn't even get to court anyway as the prosecution service will try and look at this objectively and for what it is,which is a girl trying to blackmail you over an image she sent and is then trying to extort something from you. But it's crazy days and the law can sometimes be bound to act regardless. Get a lawyer and pull her down if your going down, just make sure she falls further and harder. If your in the UK the citizens advice service can be very helpful and might be able to at least get you some legal advice as a start. Good luck