Can a girl who sent her pictures (with and without the niqab) to other girls via WhatsApp be forgiven for her sin, and is she sinful if foreign men see them?

You are not sinful for sending the picture if you did not intend for men to see it, because of Allah's saying: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended." You did well by requesting its deletion, and nothing more is required of you.