Location is everything. Choose a good spot for the image you have in mind.
As most images will have a ragdoll, get it situated in the 'basic' position you want it in. Also, notice the faint yellow light. That can be useful for not only seeing the models you pose, but in this situation it looks like an inside car light.
It's important to, if your ragdoll has exposed eyes, use the eyeposer. Without it your ragdolls will have a thousand-mile stare.
Some models will be able to have bodygroups/skins changed. It can be useful to spice up the image, giving it a more unique look than a default model.
Add some lights if you dislike a shadow that looks off, or if you just need something to look highlighted. Also, the model change that happened to the man in the passenger seat- it's ok to change your mind about something in the picture. Take your time and make sure you like something before heading to the next thing. Also, another thing to add here, this model doesn't have face posing, and there's a decent amount of models like that. While they're not ideal, it can be useful for a more HD model. Just make sure to pose the eyes.
Back to enviroment- there's a tractor off in the corn field next to the van. It's lights are on, but the light it's producing isn't dynamic. Lamp are however, so it's worthwhile to spawn in some lamps to give it a more HD appearance. PRO TIP: The lower the FOV of the lamp is- the sharper the shadows.
Two things to mention here. For images like this, positioning is everything. (If you're not careful, the hard work on the zombie in the background would end up as an arm or a portion of it's head. Two, you may notice the shadows beneath the model that don't add up with the shadows form the tractor's lights. There's a mod that allows you to remove default world shadows from models (Toggle Shadow Tool, I believe). However, as the zombie's legs won't be in the final image, it won't make much difference.
Here, you'll see the image starts coming together. One thing I did here was zoomed the camera to allow focus on both the zombies and the males. (To zoom, hold down the right mouse button with the camera equipped. Also, moving back will allow you to zoom in more, decreasing the distance between foreground, middleground, and background). I also decided to move the left zombie to the other side, as I had difficulties getting both zombies to fit in the small window of the van.
A very important tool to use is Color Modification. (Found in Post Process>Shaders).
Before
After
Alt. After
Probably the single-handedly most important tool you can use. Super DoF allows you to click anywhere on the screen to focus on, and depending on how much or how little you blur the image, can either give a soft haze around the focus point, or make it look like the world is spinning around it.
That's about it. Hope some of this helped. :>