In response to the previous comment, here's my interpretation of this cool painting. The guy in this painting has his hand raised as if he were saying: "I alone shall conquer this underworld by killing or enslaving all the demons within it!" And the skulls, they are of those who have fallen, although they nevertheless maintain their ability to communicate silently and to sense what's going on around them. These two particular skulls probably see the guy at the entrance, and their silent communication is this: "Let's see how far this hero advances before he becomes one of us." The skulls are immobile, but they can see whatever others skulls see.
How about some follow-up paintings of this same guy further into the depths of the underworld?