It is much easier to edit images posted unmagnified. The forum has a built-in zoom function.
I made an edit, but the proportions got a bit skewed when I tried resizing it. I fixed it up a bit manually, but it is not exactly the same.

I changed it from 15 colors to 8. I didn't change the silhouette (on purpose, anyway), but that could also benefit from some attention.
Basically your big issue is banding (those strips of colors lined up next to each other in the shading.) And you're shading as if these are 2d pieces of paper rather than actual forms with heads and noses and protrusions. Just think about what would block the light from hitting what.