Will a girl be held accountable if a young man sees her naked pictures that she sent him, and he does not delete them?

The questioner's regret for her heinous act indicates her righteousness, and this is one of the conditions of repentance that requires fulfilling the remaining conditions. She must ask the person to delete the forbidden pictures. If he does not, then there is no sin upon her, for the one who repents from a sin has his repentance accepted, and the persistence of the effect of his sin does not harm him. She must cut off her relationship with this man and other non-mahram men, and she should make this a motivation for rectitude by acquiring beneficial knowledge, performing righteous deeds, and associating with righteous women.