Image from Elostirion.com
Ringer Grond sends in this well thought out analysis of the above image:
I think your first explanation must be correct. These could be the dead men of Dunharrow, but I have my doubts. Firstly the crowns and rings would have been appropriate for Nazgul as they were all kings and they all had one of the nine. Not all of the men of Dunharrow were kings, though this in and of itself doesn’t disprove the theory as they probably had at least two.
I would offer a theory of my own. I feel that these are the Nazgul as Frodo saw them on Weathertop. While he wore the ring he could see them “in the flesh” as opposed to the rest of the world that only saw negative space clothed in robes.
When Frodo put on the ring he entered their world of shadows and became aware of them. They also gained a larger measure of power over him.
Shortly after the fight, Frodo asks about the “Pale King” that stabbed him. (I don’t have the work in front of me or I’d quote, sorry) This of course being the Witch-King, but seen as his old self only twisted by centuries of wearing one of the Nine. This theory might also explain the lighting.
These are very compelling images of what the Nazgul might look like in a physical form to my mind. I could of course be dead wrong, but we’ll have to wait and see.