On the graphics front, I'm still going to stand by my position. Again, while graphics aren't the be-all-end-all for games/mods, they're what generates a first impression and if your stuff ends up looking visibly worse than other people's stuff, people will go "meh". Good graphics also make people overlook deficiencies in other areas. Anyhow, my advice here isn't to totally disregard everything else, it's that graphics should be your highest priority when putting out your initial product (as it's what's gonna convince most people to pick it up).
If you want an art critique, the silhouettes, detailing, and all-in-all general design are quite good already. If you want to improve things however, there are two big things that stand out: the color balance/use of true greys and the lighting (or relative lack thereof). For color, you should try to never use true grey colors (100% desaturated), use greys that still have a little bit of color in them. It's really hard to quantify, but gives things a lot more "life". The colors could also stand to be a tad bit brighter (but you're totally welcome to disregard me on this one, could just be my laptop screen). As far as lighting goes, right now it looks like theres only a very diffuse light source directly from the top... if that. Starsector sprites (modded and vanilla) all have a distinct light source that is above and slightly in front of the ship. Furthermore, the highlighting and shadows are barely there on your sprites - there is almost zero dynamic range in terms of lighting. You can add some convincing lighting pretty easy in a tool like Krita or GIMP by painting over white/black in a soft-light layer on top of your existing sprite. Also colored lights (if you want to go that route), work reeeeaaaallly well to establish depth in areas with heavy shadow.
So TL;DR for art critique - don't use true greys, beef up the lighting a crapton. Don't like the pirate sprite paint, but that might just be me.