Having not seen your bases, it\'s tough to say for sure. However, if you have any ground showing not covered by skulls or if you have any other non-skull objects mixed in, you can cast the light from the skulls and illustrate the glow effect that way.
If the bases are entirely skulls, I think you\'ll have to do an internal glow effect (like DR described from the eyes, nose and jaw) or cast the glow up at your figure\'s legs/etc depending how intense you want the glow to be.
Does that help?