Poor duckies
First off: I want to say that that was freaking funny.
As for your problem with the arm: why don't you create a second sprite of just the part of the arm and hammer that is obscured by the body during those two frames and flash it on the screen to complete the picture? Hope that helps.
Also what you might try separating the shoulder from the body (copy not cut) and placing that over the arm over the rest of the body. That way you don't see the joint and the arm moves properly over the rest of the body.
More please,
Songpenguin