@Background Pony #69A1  
Wow. Someone with a DeviantArt account should post this where the artist could read it.

Artist: in my opinion the background trees look weird because they’re too bright in color and too uniform. They look flat and cartoony–and really need to be in shades of green that are both darker and less saturated, so to speak–duller, darker, a bit more greyish, so to speak. Olive drab, Feldgrau, and “Foliage Green” were not just chosen at random for camouflage–they are colors widely found in vegetation, especially in temperate areas. Also, for camouflage, the human brain seems to interpret areas of duller, darker colors as shadows, causing the “software” to skip over those areas in favor of looking in the more brightly lit areas where it “thinks” it will more easily be able to see. In my opinion the background looks weird because the dominant shade of green is too saturated, too light in shade, and too purely green. It is the green of well-fertilized, immaculately cared-for golf course grass, of expensively maintained lawns.
 
The color you are using looks close to #20c020, if you’re using hexadecimal color codes. #669900 and #646c48 would look more natural and more appropriate.
 
Also, from a color-theory and composition perspective, bright colors draw the eye and make the brain say “this is the center of attention, this is what the picture is about.” Dim the bocage, dull it, darken it, fade it out. The mare and the tank turret hatch are the subject. Bring the eye to them, make them the focus.
 
Secondarily, and this begins to get into areas of how complex you want to make the background, trees and brush aren’t a flat two-dimensional green wall. They’re three-dimensional objects with highlights, shading, and shadow, even when they’re so far into the distance that you only perceive them as a green sphere or cone on a dark stick coming out of the ground.
 
Disclaimer: this advice is worth what you paid for it.

Michael Wittmann?

@Alex Vostox  
https://www.youtube.com/watch?v=J-wS5xviDvs

PANZER MARCH!!