What happens if a 17 year old sends me a nude photo but I didnt keep it?

This sounds like the beginning of a common scam on the Internet. There is no girl, there is no mother or father -- only Internet fraudsters.
Do NOT send them any money. They are trying to blackmail you, and you are the victim here - not some young girl who does not exist.
Block them and do not communicate with them in the future.
I have never heard of these fraudsters actually posting the pictures because 1) It takes time and makes them no money, and 2) It makes it too easy for them to get caught.

Technically, you could be charged with Receiving and Possessing Child Pornography, even if you don't still have the photo (although without the photo, it's hard to see how the police could make that charge stick.
HOWEVER, don't be too sure that the picture was never saveed anywhere. Once something is posted to the internet, it NEVER really goes aways.