Haven't sprited in a while..

[Ego]

The animation may seem unsmooth because you don't have any stable points. In something like this, I would recommend that one of the feet stays in the exact same place. You have him arbitrarily shifting every so often. Here's a visual example of what I mean.
->
All I had to do was shift your entire first frame 1 pixel left. This makes his left leg consistent and stable through the whole animation.

He looks a tad jerky when he moves his body, and your gray for the skin is probably WAY too close to white. Otherwise, nice job, I like it.