I was watching SP in the US yesterday. The picture that the mystery perosn was holiding of Saloni was the picture of Saloni prior to marriage. He had this pciture on his desk. She was wearing the same dress that she wears in the opening credits when she is shown with Neel.
Even though Kunjan knows her before marriage,also, I don't think that he obssessed with her that he will put up posters of her. He just wants revenge. He is not in love with her. The mystery person is Neel because he was in love with her. He wants her back. The only way that she will come to him is by breaking all ties with Nahar.