If your talking about the guy underneath the armour then I don't think there is any pictures at all of what he looks like.
The game designers wanted to keep it a bit of a mystery, and I think that they wanted you the gamer to create a character underneath in your imagination.
I suppose this is better than master chief taking off his helmet and turn out to be some uncool ugly person who disappoints you.
But since Halo has been such a hit the producers of the game will probably produce another sequel which will hold many mysteries maybe the un helmeting of the Master Chief himself... who knows. Halo Wars probably holds the face of him, shaved head, and a small goatee.
However, some claim to be able to see his face in a Halo 3 trailer. While this is disputed, the alleged face has no pupils, a scar under the left eye, and very pale skin. Some claim this is just a reflection of ships in the background, and Bungie refuses to comment.
-edit- The chief had brown hair, freckles and a gap between his front teeth at age six.