Should a girl marry the young man who blackmails her with her pictures and she goes back to talking to him to prevent him from sending the pictures to her father, or should she let him do whatever he wants?

It is recommended to supplicate and seek refuge with Allah for protection from the evil of threats and to recall Allah's punishment for those who repent. As for marriage, having a former wife is not a flaw that prevents marriage. Rather, what is important is that the suitor be a person of religion and good character. It is preferable to perform Istikhara (prayer for guidance) before making a decision.