your doing good man! was gona say you need more values studies but jatherip seems to have mentioned it already. when doing them tho make sure you use all the tones from dark blacks through to light greys. will help you keep depth in your work. anyways keep it up man!
your grey-scale environments seem to be the most successful at the moment.
looking at your 'skull from memory' I see you need to study skull structure a little more - also you have a similar problem to me, I have trouble maintaining symmetry - remember to keep flipping your image as you work, also a simple vertical line drawn down the centre can really help.
you are progressing nicely, my suggestion to you do more photo/ life studies and less from Imagination. once you do tens and even hundreds of life/photo studies the imagination will not be constrained by technique and at some point you will see improvement from painting to painting.
so what are you waiting for? hit those reference sites!