What is the reason that some put up a picture of a tzadik, I know of the midrash about Yosef seeing Yaakov’s face, but is there a reason and did any rabbis say anything about it and if it was a picture about themselves?

The idea of that midrash, that when picturing Ya'akov's face, Yosef didn't dare sin with Potiphar's wife, is actually a pasuk in Yeshayahu (30, 20), "And your eyes shall see your teacher". An additional explanation, is that when one constantly sees the Godly and kind countenance of his teacher, it serves as a reminder and declaration of his ideology and what's of primary importance in life, not to be confused by secondary, mundane issues. Rav Kook had pictures of the Vilna Gaon, and his adversary, the Ba'al haTania, founder of Chabad chassidut, davka right next to each other, declaring that it expresses his holistic and unifying ideology! It's sort of like the mezuza and tzitzit, but in the olden days, they didn't have photographs. Most rabbis today opt to overcome their natural humility and allow these photos, in order to offer an alternative to others, who lehavdil, hang posters of their favorite athlete or movie star. We also want to stress, especially to our children, who the "stars" really are, and that there are still tzadikim we can emulate, even today in the modern world! That our ideology is "do-able".