hi there, very nice job.
i'm a noob (and not actually an artist), but may be you'll find some of this useful:
it seems that you are using 3 different kinds of perspective for the tent, for the fireplace and for human/zombies.
there's no ground shadows, including shadows from the fireplace lighting. check this for example:
http://www.heart-machine.com/wp-content/uploads/2013/11/HLD_Screenshot_01_camp_1080.pngmaybe you can use less straight vertical lines at some parts of human's sprite (doesn't looks very good, imho)
i'm not sure trees/bushes like this(detailed and contrast) are conforms to the ground/grass like this(which is just filled with color. is it just mockup?). trees are textured, but still flat to me.
lightning on zombies bodies doesn't make sense to me.