wasn't happy with the one i made for the excercise, that's why i made a new one to see if i got better.
since i am currently no citizen i can't submit it for the excercise but still wanted to show the new one here.
for bigger resolution:
http://sypr0.deviantart.com/art/23-Rock-615222611
for the ones interested here is the old one i made for the excercise :
https://cgcookie.com/exercise-submission/painting-on-the-light-exercise-2/
Great improvement.