Is repentance possible after possessing forbidden pictures and then deleting them? And is it obligatory to seek forgiveness from the owner of the pictures, despite uncertainty about her identity, to avoid returning to the sin?

The door of repentance is open until the Hour (Day of Judgment) is established. The questioner must repent, regret his sins, and resolve not to return to them. If his repentance is sincere, then Allah will accept it and pardon his misdeeds, and he will become as one who has not sinned. He must completely sever his relationship with the girl and not communicate with her.