From an animation standpoint the one thing that stuck out to me is that the eyes go bad and the mouth opens at the exact same time. What you should want to try and go for is what's called overlapping action, meaning that not everything moves at the same time. It creates a weird, static look.
Here's a quick edit, simply adding one frame where the eyes go evil before the mouth opens.
A question for you: Is the skull supposed to be looking down as his mouth opens? The shading and movement of the whole skull suggests this but the eyes stay in the same spot which means that it's not really looking down. To change this you'd only need to move the eyes and nose hole down a single pixel.
12 Principles ofAnimation Check this out for quick reference.
Richard Williams The Animator's Survival Kit Check this out if you really want to get into animation.
EDIT: Also, welcome to the forums!