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
@konrad01 got it pretty close, the Buddhas symbolize various ways Dharma is presented in each of the six realms, expressed so as to match mental disposition of the realm's beings. If you look carefully, on most depictions of Bhava-Chakra the Buddha in each realm holds a different object in his hands. That is the key.


in Hell realm Buddha holds a cup of water, because for Hell beings the point of Dharma is to Escape from Suffering. This is the only seed of positive thinking that exists in their otherwise 100% negative mind.
in Animal realm Buddha holds a book, because for Animals the point of Dharma is to acquire Wisdom (and through wisdom, control and power -- which is what they secretly crave despite their laziness and inertia).
in Preta realm Buddha holds treasury, because for Preta mind everything looks very desirable from a distance but once they get it they realize that it is empty, so Dharma for them is the best hope to finally get something of Real Value that won't turn out to be yet another lie.
in Human realm Buddha holds the begging bowl, which stands for compassion and generosity. Because in Human realm the main feature of mind is obsession with quasi-valuable (such as the "I"), developing compassion/generosity is the best way to liberate mind from egoistic tendencies and obsessions, something required for Enlightenment.
in Asura realm Buddha holds vajra I think, or some kind of sharp weapon, symbolizing Perfection of Excellence. Because Asuras are very sharp and competitive, they are incurable perfectionists, so Dharma is presented to them as a way to perfect their excellence, until they eventually realize that the highest perfection is to transcend their perfectionism and thus attain liberation.
in Heaven realm Buddha holds flower as symbol of impermanence. Because life in Heaven is so perfect, the only potential problem Gods have is entropy finally kicking in. Impermanence is presented to Gods to remind them of their vulnerability/susceptibility and Dharma is presented as ultimate Protection from Impermanence.