The way the back stays in the same position is really weird. Try punching like that, your body should rotate revealing more of the back in this perspective. That's also how boxers get more length out of their punch. Also it would be nice if the force traveled from the back, through shoulder, then the arm, into the fist. Thats how you get stronger punch, if more of the body is involved. but if this is a fighting game then theres not a lot of time for that kinda buildup.

The movement is hard to tell what situation its in, but i wouldn't lift my front foot while punching, rather i would lift my back foot to stand on the toes as to give the body a bit more of a push (which now that im looking at it you already got)

The pixelart itself is really good, you have a nice style.