I use photoshop and lots and lots of: layers, use of smudge tool, use of blur tool, use of feather brushes to get the gradient shades of dark to light. I experiment a bit at a time using separate layers and then I build layers upon layers until I get what you see there. Oh, also I start the entire thing out first by using the lasso tool and just the color black...to block in shapes. I get the shape of the pieces right first (a silhouette of, for example, the helmet) before going in and doing color and gradients...helps a lot to get the basic shape right first.