Agreed with what Arc said, when creating a logo always consider what it will look like at a small size (like on a pen) and see if its still recognizable.
To better achieve this, try vectoring out simpler shapes of the skull, right now it looks like you image traced it and filled in the highlights and shadows with a pen tool, try to simplify and vector it.
Try using this image as a reference. This person defined his logo with thicker black strokes. He then subtlety highlighted parts of the skull with (one color) lighter grey.