If you're still open to making changes to these, then it's too early to submit them to the PJ gallery, which is meant for finished works xP Submitting to the gallery and posting in the WIP section sends mixed messages to the mods.

The dithering is making the skulls look very textured, whereas skulls are usually smooth. At this size, I think you'd be better off using flat colours without dithering, so that you have more room to clearly define the shapes/forms. The dithering creates ambiguity.
As a general rule, I think it's best to avoid areas of dither where you only get 1-3 rows of dithering, because it doesn't read as a solid colour in those cases.

The highlight on the lower jaw seems to come from side lighting, while the highlights elsewhere all come more from the front. I think it would look more cohesive to have the highlight be on the chin rather than on the side like that.


The blue glow has some banding. Try to avoid letting pixels line up like that, as it emphasizes the pixel grid and makes your work look pixelated/blocky. Stagger the shapes instead, and/or make them wider than a pixel thick, so that the lining up is less obvious.

The glow/flame's wisps all ending at roughly the same level looks artificial and boring. Have some of them (probably the middle ones) rise up higher, and make them all different widths. Fire is rarely so uniform. Similarly, the glowing bits around the skull all look uniform in size and spacing, vary them up some :D