1. The main difference is social ordination.
As Campbell sees it the priests are functionaries of a social sort. They are ordained to conduct rituals carry on their tradition of that ritual which the public has been conducting for a long time. The shamans as he said derive their symbolism, familiars and deities from their personal experience, their understanding comes from that not from the traditional views.
2. The caves in which the prehistoric paintings were found, Campbell classifies as Temple Caves.
Campbell suggests the paintings served the same function that our frescos and images of the saints serve, but we represent our spirit world in a human form while the prehistoric person saw the spirits as and depicted them as animals. They share the same message, the same transformation of consciousness according to Campbell.
3. The answer here are artists.
Artists are the modern mythmakers according to Campbell. He states that they are the modern mythmakers of our time. The shamans were the ones who turned inwards for knowledge and who are open to the song of the universe as Campbell calls it. The artists share their knowledge and ides with the people and create interaction between them.