The latest edit is certainly looking better.
this is my final edit of the skull before im going to leave it as it is.
I'll try to make it quick then.
Perhaps I've went a bit too far ahead with my previous suggestion, as it would require you to already have some knowledge in light and form in order to apply. Also, despite the fact that I view it more as a mental exercise, something you don't actually draw but imagine in your mind while you're drawing, which in itself you get a feel for after a while; I think that it would be useful for you to practice and study some basic shapes (cubes, pyramids, spheres, etc.) in order to start getting the hang/feel for light and form.
It goes without saying, but you're not going to improve overnight, so don't give up if it doesn't look good in the beginning.
I'll approach this in a slightly different way.
Notice where I placed the highlights in the edit in my previous post, they correspond to where the light is at its strongest; or in other words, where the skull is perpendicular to the light. A good way to show this would be to take a profile of a skull, and extend light rays from the direction that light comes from, as shown in image 1; I've marked the areas where the skull is perpendicular to the light in red to illustrate better what I mean.
The same is true for shadows just the over way around: the less perpendicular the skull is to the light, the darker it'll be (this is without involving reflection in the mix however).
In image 2 I just wanted to demonstrate that the further the object is from the light the darker it is, even if it's on the same plane,
however, the contrast between the darkest and lightest spot on a single plane, as shown in the example, is determined by the size of the light source. So if we take the sun as an example, the difference is going to be quite minute; if we take a lamp for example though, the difference is going to be very visible.
In image 1 and 2 the difference is closer to the one caused by the sun, with image 1 being an even better representation.
The problem arises when you want to depict a 3 dimensional object in a 2 dimensional space, and that is solved only through practice and study.
I do hope though that this perhaps helped you, or will help you, understand light and form a little better. The problems in your skull are not so much related to pixel art as they are to artistic ability itself.
