I should have been more clear on the various factions. Chaos is the only 'race' worthy of doing five different portraits for. There is Chaos Undivided whose main champion in the 40K universe is Abbaddon the Despoiler, the right hand 'man' of Horus the Betrayer whom died in battle with his father the God Emperor of Mankind during the Horus Heresy. A seven year long battle that tore the Imperium of Man in half, pitting brother against brother, more than ten thousand years prior to the 40K setting. 40K literally means the years are in the 40,000 AD/CE range.
Chaos is the unifying reality of the four opposing Chaos Gods. There is Nurgle, God of Disease, Decay, and Destruction; His Legion is the Death Guard. Then there is Khorne, God of Blood, War, and Murder, His Legion is the World Eaters. And Slaanesh, God of Pleasure, Passion, and Decadence; His Legion is the Emperor's Children. Finally, there is Tzeentch, God of Change, Evolution, Intrigue, and Sorcery; His Legion is the Thousand Sons. Each God is bent on warping reality to their ends using their own special brand of evil.
Now, each of the four main Chaos Legions has their own leader and champion that any 40K enthusiast would recognise. However, for all of them, except Chaos Undivided, I think the images you can find of the four gods would be more interesting as race leaders.
Most of the other races I mentioned: Necrons, Orks, Tau; don't have any such divisions. The main exception to this are the Eldar, whom have like sixteen factions but only the two major ones 'Eldar' and 'Dark Eldar', which I mentioned. Hopefully this will help you understand our requests a little better. If you want more info on the who and why, just drop in a post.
Really looking forward to the Necrons. Cyber Space Ghouls/Zombies are scaryawesome! Thanks again!
Edit- And if you really want I can even harvest all the image links and post them up for you, let me know.