First with the chess board. I can clearly see the dimples on the white squares but can barely see the dimples on the black ones. Maybe a little more contrast there?

Also try some anti alaising on the diagonal markings on the sides. I haven't tried it meself but I think this next method should work. You take the rgb values of the lighter grey (123/125/115) with the rgb values of the almost black colour (16/20/8) you add each seperate red, green or blue value and then devide them by two. (123 + 16/2, 125 + 20/2, 115 + 8 /2) and use that new colour to smooth our the pixelated lines to make them appear more straight instead of a clear pixel line.

The bat looks really great. It looks a little flat though, but I really like the wings. Soem of the lines on the wings don't go through the entire wing but stop a few pixels before reaching it though.

On the pirana plant. I look at the stance and the legs and this assume it is meant to look human. But the arms are bend in the actual warm, rather then going down from the shoulder. That doesn't bend, it merely has a long range of motion without actually bending. The same with the elbow. Just take a straight corner instead of bending it in the animation of the shooting arm.

If you got that right, maybe make the entire body react to the gunblast. But that can be hard to go though.

I like them :) keep up the good work :).