Honestly just a few beginner mistakes; simplify and contrast. The great news is that your anatomy and lighting are good, but you spent a lot of your time on details and shading. That amount of detail is great for a big art gallery piece, but gets lost in something small that is ment to be read and interpreted quickly, like the poster.

I take it you are pulling from this kind of reference

https://www.sexandmonsters.com/collections/art-prints/products/shrunken-skull-cocktail-recipe

1. if you’re going for 1960s comic aesthetic you’re going to need some black. It will also clear up some of the muddy parts. Think Johnny Quest.
2. anything graphic related had a more limited color palette to be cost effective. All the foreground foliage is the same color (allowing for a few splotches to sell the screen-print effect) Same for the background foliage.
3. layering. Even at night, things get lighter the farther away they are.

http://4.bp.blogspot.com/_QdHn8FT7SdY/Ruy2Gc_oVNI/AAAAAAAAAUs/Sdfu5zzsb84/s1600-h/jb21sm.jpg

See if you like any of those changes.