Thanks @Faraleth 
I am not sure about the resolution you should use, because Starbound is a pretty funky game to mod up when it’s about skins. Pistols in starbound seems to be something like 15 pixels lenght, but there’s no way you could fit an Empire-9 in a 15px square.
I’ll base myself on my first test, trying to make a Blishlok pixel art : do it in a 3010 square, but don’t hesitate to change the resolution if it can make it look better. My first attempt at trying to make the Blishlok ended with a 3111 image size. Maybe avoid going above 45px in lenght and 20px in height. I’ll then try the skin in game and see if we can go for a bigger image.
The weapon should look like it was shown from the side, pointing to the right.
Here’s something handy : concept arts of each weapons ! 
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http://hydra-media.cursecdn.com/dirtybomb.gamepedia.com/thumb/5/57/Blishlok.jpg/600px-Blishlok.jpg?version=42e77f9de9563138429fa4444001bd19
[/spoiler]
In addition, make sure that the background is transparent. If you can’t, well I’ll try to do it by myself so don’t worry too much. Of course if you find this fun, don’t hesitate to make more weapon pixel arts. Maybe I’ll be able to create a big “Dirty Bomb mod” for Starbound. 
Here’s one of my attempts at trying to make the Ahnuhld-12 : http://imgur.com/4dDoDlr,bvKk5G2#0