Who are the Buddhas on the Wheel of Life?

In a lot of images of the wheel of life there are Buddha figures in each of the realms. You can see them in the image below of the full wheel of life and I've also put in a close up of the hell realm with the Buddha figure clearly visible. So a Buddha is in hell, animal, hungry ghosts, human, heaven and titan realms. Who are these Buddhas and what are they doing in each of the realms. What is their significance? **Wheel of Life** **Hell Realm with Buddha on the right**