I would not get too concerned with mold in the attic based on that picture. It is pretty common when eave vents are missing or are blocked .....or if there is inadequate ventilation. I would ensure there is adequate ventilation (1 net sq. ft. per 150 sq. ft. with 50% at the ridge and 50% at the eaves). I don't even recommend installing the fan-- I don't think it is necessary and have read that the fan can cause other problems in the home.

Just my take.