In my opinion, itisnt bad after all. i dont know exaclty what sort of species is this skull, but if you created it from your mind its your idea. Simple human skull has special shape, try to look at michaleanglo first drawings you will see the shape.
Try to avoid of useing black colour as a part of the drawings. Deep black colour is always a hole in the picture. If you would examine color of the shadow of the things youwill see that its always have some sort of colour. Deeper darker colour the deeper hole will be looking.
Hope my word are understandable, im still practicing.
Ive made quick edit of your work, and some palete orders to use less colours. Id used GIMP
Its not perfect but itll help you get something clear that i was talking about.