The St. Louis' historic Old Cathedral, known officially as the Basilica of Saint Louis, King of France, is one of the prettiest churches that you will ever see. It is situated near the downtown riverfront very close to the Gateway Arch. The original log chapel is amazing to see. There are also some photos in the church and paintings as well.
Definitely worth a stop if you are interested and going to the Arch grounds anyway. Very great place for pictures.
Parking is limited by the church so you may have to park further away and walk.