Will the sin of a girl be forgiven if she repents sincerely after a young man lured her and took pictures of her, and will she be held accountable for his viewing of the pictures after her repentance?

Satan exerts his utmost effort to lead humans astray and tempt them, especially concerning the relationship between the sexes, and Allah the Almighty has warned against him. You have done well by initiating repentance, so you must be cautious about being lenient in your interactions with non-mahram men in the future. It is forbidden for the young man to look at your picture, and you should try to have the picture removed from him in a way that does not lead to a greater sin.