The sprite itself looks good, but it lacks a little detail on the chest.
While he's running, his shoulders seem to bulge away from the actual body a bit too much, try to concentrate the main movement on the arms.
His legs are a little stiff and seem to be too far away when he's running, while his standing animation in the top row is more accurate I think. For now, it looks like he's hopping from one foot to the other.

But overall, it looks really good. Better than anything I could do. =) Keep it up!

The arms only have two positions each. This makes them pop back and forth. They definitely need inbetween frames.

The legs arent entirely convincing either. The bend back looks alright but moving forward does indeed look like he's hopping. I dont have time to try at an edit right now, but my guess would be to move the leg one pixel further up when its bending back and one pixel further down when its coming to the front, and maybe show a bit of the bottom of the foot on the furthest forward frames.

also, the top of the head has those little 'horn' looking things because its got that jagged ¬ shape of 3 pixels. You can remove that middle pixel and make it smooth