Ok well I got a bit carried away with an edit...

The stuff in front of the car is supposed to be broken glass, not texture.
There's also a couple of bullet casings on the ground but I think they need their own colour to be recognizable.
Here's a comparison for "stronger lighting" amongst other things:

A few key points here:
- You have a stack of colours that are VERY close to each other that you really need to reduce them (just the ones that can hardly see the difference. Set image to index mode or however you want to inspect your palette. You can add a black strip to the side of the image of find a space and draw the palette in 2x2px blocks if it helps.
- Try not to put subject matter direction in the middle vertically, so I moved it off and adjusted the horizon so we're more at eye level of an observer instead of a guy on a small ladder.
- Your lighting is what we call volumetric. This suggests there is particles in the air such as dust or most commonly moisture in the air. Such lighting would mean the background will fade more into they "grey" (a hue that looks good, not plain grey). This doesn't mean you can't have subtle volumetric lighting but I hope you can see from my edit it is not your only option.
- Used lighting to isolate points of interest.
- Zombie shirt is warm colour to make him stand out
- I just couldn't operate with that dude's wacky anatomy so tried to blend it into a style I'm more comfortable with.
- Dunno what the story is with the bike so I just moved it somewhere that helped with the balance and forgot about it. In my edit the second street lamp is still too high for perspective, forgot to fix. Car's front wheel looks a bit small too

I started editing colours but its too late to continue...

Maybe all the green hue is washing out the zombie a bit, maybe it adds to the feel I dunno.
Either way, work on the palette and try to make some interesting ramps rather than so close to grey and white.
Again, you have too many colours that are close to each other so fix that too...
You could turn this into a really great animation loop if your interested in pushing it.