Maybe something like this? 

Try a "blot in the basic shapes and then refine" technique. That somewhat worked for me. Keep in mind that the people viewing the art (I assume the people that will be playing the game) will probably not care about a pixel that is off and will be more focused on the gameplay and animations. That being said, this seems like a relative trivial think to stress over.

Those screens have had a lot of filters applied to them, just pointing that out. You won't see that effect from just the pixel art alone.