Well, a lot of the movie is up to interpretation. You could just see them as an '' sign '' of what will happen Liza and John after being trapped there for a while. Since Fulci was catholic and presumably Sacchetti as well it could be the souls in stuck in purgatory waiting to enter heaven. It's kind of an half-assed since I haven't delved into thinking about it very much but that's just my spur-of-the-moment 2 cents.
I'm not sure if it was the Tibet Book of the Dead, but in the movie Jacob's Ladder, Tim Robbins is looking through a book and there's an identical illustration of that cavern of Hell along with the bodies lying on the ground.