I did a paint-over to show where you can take this image. Some of the things I though needed to be pushed much further was the fire and the dragon's size (remember, fire is hot and bright, and you need to convey that to the viewer). The one, really big problem that needed to be addressed in your image was the fact that the knight was not in the dragon's line of sight. This creates an off balance, awkward scene. I tried to correct that as much as I could without redoing the entire dragon's head.