Read the guide here but found no answer to that question.

I wanted to add a deck to the Human fighter.

First I added a .png to the SHIPS/TOP folder.
So that I have there a N60_0.png and a N60_1.png.

Then I opened the ShipsBody.all file with notepad++ and changed :
Decks 1
Img "Ships/Top/n60_1 ...