As you are probably already aware it is now possible to customize your ship by painting decals as you like on your ships. It is also possible to include your custom made decals and here is the short guide for it using free {LÄNK BORTTAGEN}paint net tool but you can also use any other image editor(like photoshop, gimp etc.) but you need to export the image in .dds format. Here is the guide for {LÄNK BORTTAGEN}paint net:

{LÄNK BORTTAGEN}picture guide

1) Open the image you want to use(drag and drop into active window).
2) Create a blank canvas(new file) with 256x256 resolution.
3) Select new canavas and select the whole (white) background, you can do this with ctrl+a shortcut and press delete to clear the background (result is empty image).
4) Select the image from first window and paste it into blank canvas. Select "Keep canvas size".
5) While holding shift select right bottom corner and scale your image to fit the canvas width.
6) Hold the left mouse button on the image(but not in center) and pull it down to fit the middle of the image.

If you want your image to have reliable coloring in game:
7*) Select Adjustments -> Black and White. (this enables you to reliably color white surfaces in-game).
8*) Select Adjustments -> Invert Colors. (this enables you to reliably color non-white surfaces in-game).

OR

skip 7*) & 8*) if you want your colors to stay true to the picture but it will be harder to color it in-game (prefered selected color is white, other colors might seem awkward with this mode).

9) Save the file as .dds format.
10) Put the .dds file/s into "GameRoot\Data\Textures\Decals" (located in the game install folder, usually "Steam\steamapps\common").
11) Start the game and apply your decal on the ship in-game and PLAY with it!