Symbolism, Imagery, Allegory
Pro tip: if you ever see a figurehead shrouded in canvas, get the heck out before it's too late. Not only in he spooky, but Aranda's skeleton gives us a lot of information about how events went down prior to Captain Delano boarding the San Dominick. Mainly, Aranda's prominently displayed body clues us in to how he was perceived among the slaves. As a slave-owner transporting people to sell, Aranda's pretty clearly seen as the bad guy. However, during the rebellion, Benito Cereno's life was spared… he was either weak enough to manipulate easily, perceived as a good dude, or some combination of the two.
But Aranda's corpse isn't just a way of characterizing our man Benito Cereno. It's also a way of characterizing the story "Benito Cereno."
How? Well, the words underneath Aranda's skeleton say, "Follow your leader." That's a pretty strange message to send to the rest of the Spanish sailors, unless it's an overt threat that they may be following Aranda into death. Alternatively, Aranda's skeleton and the message below it could be viewed as an overall critique of authority and the slave trade: the world in which "Benito Cereno" takes place is one where the leaders to be followed have all the wherewithal (and smarts) of a rigged-up corpse.
Let's not forget that stringing up Aranda is a grisly hat-tip to the history of lynchings. Typically, slaves were punished (and set as a chilling example for others) by being lynched and left hanging. This time around, the rebelling slaves are leaving a white slave-owner up as a reminder not to fall out of line. Powerful symbolism there, Melville. We'll put this in the Symbolism Hall Of Fame alongside your White Whale.