You might want to check out my other guide on tweaking the game files :

BASICS

To take screenshots in your monitor’s resolution, hit F10 by default. To take screenshots in supersampled resolution, hit Alt+F10 (sometimes Ctrl+10). Supersampling will render the picture at 16 times your monitor’s resolution (by default) ; that is, 4*width x 4*height. With a Full HD monitor, supersampling will render at 7680×4320. Supersampling is only available in Solo and Private Group modes, and will most likely freeze your game for a few seconds, depending on your hardware. Avoid it in combat !
Mac key bindings: Fn-F10 (regular); Alt-Fn-F10 (Supersampled).

You can force Elite to take supersampled pictures in 4K only by following these instructions : open GraphicsConfiguration.xml, located in your executable’s folder (make a copy first), find the HighResScreenCapScale tag and change its value to 2 (by default, it’s 4).
Elite saves in .bmp format (more on formats below), in Pictures\Frontier Developments\Elite Dangerous.

To take pics outside of your ship, enter the debug camera mode by hitting (by default) Ctrl+Alt+Space (read the cautions!!). Ship controls work with the debug cam, including throttle and FA-Off.

SOFTWARE

There are a couple handy third-party softwares that allow instant automatic renaming (with timestamp and location) and converting of screenshots.

EDDiscovery
Go to the Screenshots tab of this exploration-essential software to set your preferences. EDD can also remove the original .bmp file after conversion.
ScreenShotChanger
Same options for this little screenshot-dedicated app.
Bash Scripts
Galaxyutii’s scripts for timelapse assembly and image watermarking, resizing, conversion etc. Documentation included.

FORMAT

The format is a key parameter in your photographing/editing process. You have to use the format with the most information in order to handle efficient editing, otherwise you’ll end up with awfully dirtied and messed up colors, overkills everywhere, etc. Here’s a short list of picture file formats and their relevant feature(s).

.BMP: default format used by Elite to save screenshots. Takes a lot of room, but is uncompressed and has all information.
.PNG: standard high quality compression format. Shrinks information as much as possible, without resorting to loss. File weight is much lighter than .bmp with its qualities.
.JPG: compresses information with loss, resulting in a very light file but a very flawed picture, with less colors (posterisation), reduced color ranges, blurs, inaccuracies, simplified details, poor darkness management, etc. The worst compression parameters are those of hosting services like Imgur or Facebook that literally kill your picture. On the other hand, photo-editing softwares like Gimp and Photoshop allow for custom compression rate and method, resulting in light file and often barely visually noticeable compression. Which is perfectly fine in most cases.
.TIFF: the most editing-friendly format, it retains more information than .bmp. Thus it is only relevant after editing, when you may have added information, and if you plan specific uses for your pictures. This is what I use for my tee shirts, for example. It’s also very heavy.

COLOR MODE

RGB: the most common, and the only one you should worry about, if you ever have to worry about color mode for common use of your pics.
CYMK: it’s used for printing and it works differently. Colors may suffer important changes between RGB and CYMK, so if you intend to use CYMK for printing, it’s better to use CYMK from the start. It’s what I use for my tee shirts as well, but I’m not a specialist.

COLOR DEPTH

Elite saves pictures in 8bit depth, which is fine, but limited. Don’t bother with it if you don’t seek the perfect edited screenshot. Otherwise, and in particular if you go for heavy post-editing, you might want to switch to 16bits depth in your photo editing software, before editing anything. This will give you a lot more colors to deal with. While this might not be noticeable with the eye, it will give an overall more natural feeling and will be useful for practical uses of your screenshots (prints, etc.).

FRAMING

Framing is a key element of the impact and appeal of a picture. You can have the most beautiful nebula in the background, if your ship looks awkward or is cut by the picture’s borders, it gives an odd feeling and can easily ruin the picture. Moreover, framing allows for drama in your pictures, allowing them to stand out amongst average screenshots. A careful and neatly executed framing separates photographies from screenshots (I know it’s only a game, and we’re not here to play artists ) . There are a few basic rules to keep in mind ; it’s pretty much Photo 101 and can be found anywhere with Google, I’m just hinting at things, here. Be aware of :

The viewer’s sight. Vital parameter. Usually, it’s straight to the center of the picture. Hence, in most cases, your subject will be centered. For a dramatic pic of a planet, where you want to really focus the eye on the surface and details, the planet has to be perfectly centered. Slightly on the side and it will bother the eye, as well as suggest there is something else in the picture, and the eye will lose its attention.