Introduction

Arma 3 is a sandbox game where you can enjoy in both Singleplayer and Multiplayer games, and there is even Editor that you can try and enjoy, allowing for more creative freedom.
Taking screenshots and calling it a day, is also a fun way to play, and there are some Mods that you can use in the purpose.
This article is to tell you some easy fundamentals to make your screenshots more insteresting and not boring.

Get inspired

When there are both Alpha and Omega, this certainly is Alpha. You want to have a clear idea of what you want to shoot, as it is very important.
There're a lot of ways to get inspired. You may want to shoot a beautiful asset in this game.
A film or a game may be a good reference. You can refer and inspired by an iconic scene of these.
Or, simply you can get inspired by a still image, an art or a wallpaper.

Make the theme and the protagonist

Protagonist, AKA what you want to tell from this scene.
It is not necessary to be a human or a soldier - but could be the environment, a tank which fires her main cannon proudly, or maybe a diorama of the surrounded scene of a firefight.
However, it is not a good idea for everyone to be protagonists. You want the only one or two to focus them, or the scene can tell nothing because it is messy.

Zoom

This works, but certainly boring.

Moved the camera back and zoomed in. This is much better.

Zoom is always the way to make the scene more believable. Usually field of view in games are very wide so Arma 3's, it almost is as wide as fish-eye lens.
In real life's camera, it is narrower than you think. You may want to zoom your camera than you imagine.

Use Composition

Composition that you can see in the camera view and apply them, is a good way to make the scene more easy to tell.
Our eyes and brain are optimized to see what we want to see. The composition is the way how to guide the viewer's eyes.
You may also try to make some blank spaces in the view, so the viewers won't confuse where to see.
Also, you can try to check the result in a small screen so can easily check how the shot feels in a blink.

Lighting and Colors

You cannot really see what he's expressing in a moment.

You can now, note that this is in a daytime.

Lighting takes a massive effect on the scene. The details in a shadow is really hard to recognize in a blink.
It is a good idea to have some light sources to brighten the protagonist's facial expression or some parts of the scene you want to show.
However brighten them all is not always a good idea, since you may want to have some dark and mysterious atmosphere.

Is it warm?

Or cold?

Also, tweak the entire scene's color makes some feelings. For example, you can use red-orange colors to make the scene warmer, green-blue colors to make the scene colder.
You may also want to desaturate the scene a bit so it becomes a bit more photorealistic.

Details and Effects

This may be the most important way to tweak the oveall tastes of the scene. If your scene is boring, it is because the scene is not detailed so some small details can fix them.
For example, you can place some garbage in the streets. Placing dusts and/or smokes tell some chaotic feelings and make the details to the scene.
You can also use some overlays, like vignette or dirts on the camera. Use some noise to the screen is also a good idea.

Exaggerate

A screen may clip a very moment of a scene, but that is not the only way to make a depiction. You can also have multiple time/moments into a scene, so-called Cubism.
For example, it is most unlikely to see two soldiers fire their guns in a very same moment - but you can in a still image.
Or, exaggerate an expression is also a possible solution to make it better.

The laser here is just to tell you where he aims. The line of sight is on the target properly but...

Make a bad shot instead, and it exposes the more details of the gun. Obviously it is better for a still image.

For example, if you place a soldier who shoots a target, and you line up them in 3D properly. However, it might hide some details from the camera perspective so it may be realistic but boring.
To fix it, you can tweak the angle of the shooter and reveal the gun to the camera. It may not lined up properly anymore, but makes more interesting scene without breaking the entire intention of the scene.
Compress or expand the time and depict, is also a nice way to do. You can boost a gun's RPM and have many cartridges than usual so the screen can tell how they're shooting.

I let him shoot his TRG in 750 RPM, because it is the regular RPM. However this is not cool full-auto shot because I don't feel he's doing it.

But if I boost it to 1800 RPM, more cartridges are in the shot. Which makes more belieable full-auto feelings from this still image.

In short, overdramatic expressions are the key.

Increase Video Settings

This is my main video settings.

And this is adjusted one.

This may be the most basic and obvious, but you don't need to have the best GPU you can get to make a quality shot. You just need to adjust the video settings for a few seconds and nothing more than that.
You're not going to shoot a photo 60 times per a second, so you can also limit the FPS. 3 or even 1 FPS is enough to shoot a photo, because what you actually want to have is a still image, not a video.

Closing

These are only a few of the technique, but you can use those easy-to-use ways to improve your artistic screenshots.
Have a nice war-photographing!