Lets start with the assumption that blacks are not human, and are fit only to be slaves. Then we throw in religion and evolution. Since we know that God created humans in his image, and that blacks evolved as apes, from monkeys, it is clear that blacks are monkeys, chimps, apes, whatever. Of course, only those "in the know" understand that, so when one shows a black person as a monkey, it is a coded message for those special - people who understand the natural superiority of whites, in God's image, as opposed to monkeys. Its a dog whistle, sent as a message not to be heard by most people, which would explain you not understanding the joke as the mean message that it really is.