Well another first timer for me, done with mouse.
It looks so wrong, I dunno why though
That’s really not bad for using a mouse
did you use this as reference? http://img168.imageshack.us/img168/3764/14360003nh9.jpg
using 3D art to paint over can be tough to shadow since it uses more than one light source. in this case the major light source is the sun and the ground is bouncing light back onto him from underneath. i would first cel-shade it and then add in any secondary light bouncing from beneath him