I really like the style you're going with here! The image is very readable, some places need a bit of work, I think.
The arm holding the gun looks very mannequin-like, like it's an action figure or something. That sounds kinda cool come to think of it
The helmet placement looks a bit off considering the pose, like it's on kinda slanted away from the camera. The back pack needs a bit of attention as well, it looks like it's planted into his back.
You did good saving this as a .png file but your colour-count could easily be brought down. For example, the darker green shading changes colour all over the place when a single like colour would achieve the same result. This might not matter for this piece, I'm not sure if you have a colour count limit, but it's something you might want to learn about in pixel art. Section II of this
tutorial by cure explains this in further detail.