I would not recommend using automatic tweening (or morphing, which is all it is for the most part when it comes to raster images) for sprites, for tons of reasons, but mainly because it would look like Keira Knightly's cooking (hey she said it, not me!).
Automatic tweening is fine for 3D objects, bones, IK or object/sprite world movement, things like that, because it's all down to math. But using it on pixels, with only a small number of frames, you're going to end up with smeared graphics and the time taking to get them looking halfway good, it'll have been quicker to draw them by hand or use other methods.
Think about it, a tween is the computer being given X number of frames to turn A into B. It doesn't know how an arm or leg moves, it doesn't know the difference between the sprites shirt and it's face, all it knows is the quickest way to get from A to B. Yes you can add curves and splines to the tweening motion. But a 2 frame curve is going to be a straight line any way you look at it, and the more frames you pack into a particular animation sequence, the more unresponsive it will feel to the player. Which goes against how a good morph works.
There are a few ways to do it by morphing sprites themselves, using weights etc. But honestly the results are never going to look all that great, so I'm going to help you by not going into detail on how to do it.
It might seem like a handy shortcut, but it's really not. You'll get far better results doing it by hand, or doing it in a 3D package or with vectors (both of which is going to handle it better), then converting and using the output as your sprites. And yes, 3D can be used quite nicely for a 2D sprite, even if the sprite isn't going to look 3D (cartoonish ones for example). In those cases it's pretty similar to how bones work in construct, just easier and more advanced (since those programs are made for that sort of thing) only you'll have the choice of avoiding the segmented body sections by being able to use real skeletal systems within the 3D package (but then you'll also miss out on the rag doll effects Ashley suggested for a future version with constructs bone system).