Thanks a lot for the images. I spent some hours doing a study of head, using some photos of a japanese girl, and modeling with ZBrush, and I understood some points I was doing wrong. The problem is I can draw a head in various positions, but I don't have a 3D vision of a skull in my mind yet. But I think I can overcome this practicing very much. I'll search in Google for references of skull, and then fix my model.