(I hope this is where stuff like this goes. Not a mod though.)
Since I originally suggested the Heavy Turret addition, I figured I should at least try and suggest GFX for it. Here's a little something I just made.
Inspiration: Dreadnought (brown/grey colours) , since I imagined the weapon of the turret being similar to Dreadnought's.
Size: Bigger than 'normal' turrets, smaller than Ion Cannon. (as far as image files go, small turrets seem to be 32x32, ion cannon is 256x256, but takes the space of roughly 128x128, so I based the size on 64x64.
No colour masks
or other mark versions yet. Can be added later if desired.
Let me know what you think. I've only sprited a few space things before, so I'm still noob-ish, I think.
EDIT:
Added mk2 and mk3