the problem is that i cant change the turret anymore because this tank is for a game and the turret will rotate later so that a lightsource is impossible to draw on that one.
funny, that wasn't a problem for every
shmup ever made...pixel art for games is a funny thing. ANY light source is better than no light source, but you know what would be best? Painting different lighting for each rotation of the turret. More work, but looks sweet. BUT even a rotating light source would be a vast improvement over this. You could also choose a nearly vertical source to minimize any disconcerting effects during rotation. Another compromise would be to mirror the sprite when you're rotating through half the circle, so that the lighting is mirrored (though it would not be possible with the current turret design).
Point is, there is no reason to avoid having a nice clear light source on this piece. All it does is confuse it and make it harder to see and to read. scrap those details, and start with some broad, high-contrast colors to sculpt your basic shapes. Only then should you start adding details. Also, the black outlines all over look really bad, regardless if it is a style choice or not. Those shapes should be clear and easy to understand without outlines if you paint your light and shadows correctly.
