what does the raft of medusa have to do with slavery?

The painting is symbolic and one could speculate about where the black man in the painting is coming from - or what he is trying to get away from. Maybe he was a slave and is escaping. That could be one interpretation.

"The decision to place a black man at the pinnacle of the composition would have been controversial, and was an expression of Géricault's abolitionist sympathies; the art critic Christine Riding has speculated that the painting's subsequent exhibition in London was planned to coincide with anti-slavery agitation there."

http://en.wikipedia.org/wiki/The_Raft_of_the_Medus...