So I use those zombies and you decide to whore them?!?!? HOW DARE YOU!?!?!?! But in all seriousness, they look very slapped on. In order to pull off realism, you have to match hue, saturation, and lightness/darkness perfectly with the background. Keep depth in mind too. Sharpen the images in the foreground, blur some in the background. The zombie on the left has all that fog in front of him, but then the guy behind him has nothing, when in reality, fog has the opposite effect.
