Mark Chong is a great teacher. He makes a video series called Ten-Minute Drawing Techniques and he also does online tutoring.

He's really good at breaking down complex stuff into dummy-friendly explanations, from drawing and digital painting techniques to lighting principles, and to concept and story. He can also propose exercises to improve specific skills or simply to get over one's weaknesses. From the tutoring sessions I've taken with him, he would answer whatever stupid basic question I ask with always the same patience and enthusiasm. I warmly recommend him.