I just recently read "Warlord of Mars" and I really loved the whole retro-sci-fi idea. I did some searching on the web and discovered that a noted illustrator, Frank Schoonover, was the original illustrator for "Warlord of Mars". Here is his cover and my sketch based on it. It was fun to do, I had lots of fun figuring out the gun belt and all the doodads.
One thing I was playing with the John Carter's hair style. He was a Civil War veteran, so I wanted to give him a bit more of the Buffalo Bill look.