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.png

maybe 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.

hi ovle, thanks for your comment!

Everything still is a work in progress, posting to try and get feedback as I go.  The grass is just a placeholder as I'm not even sure whether the campsite will be on dirt or grass.

I guess those 3 could be described as three different perspectives: tent-oblique; person-flat style; campfire-realistic.  Or maybe you mean it looks like the POV is from three different heights.

Thanks for the link to the image.  I do have small light on grass coming from campfire but I see what you mean by the rings.  Like I said, the grass was just a placeholder but this made me realize maybe I do need some more distance on the light from the campfire.

Some of your other comments point out general problems but not solutions or even the specific cause of the problem:
"lightning on zombies bodies doesn't make sense to me"
"trees are textured, but still flat to me"
Maybe you could say which body part looks off or the light source you perceive vs light source you think it should be.
I'm a beginner in a lot of areas so I know a lot of stuff is off or inaccurate.  So just generally stating the tree looks flat, for example, doesn't give me any direction on what to try.  But I still am happy that you commented.  Thanks!

Here is where I'm at currently, still a lot of this is WIP but feedback welcome: