What about away to group images that are on a layer? For example, if eyes and mouth were grouped to a head, but then the head is set to rotate, then everything grouped will move with the head. Not sure why you'd want a bobbing head, but I would use like head gestures, or join a joint to another joins, so that it's some kind of random noodle type movement.
Or maybe what I'm imagining is moving a whole layer of object, or giving a layer a pivot.
BUT I'm probably just overcomplicating the system, and just make a simple drawing, haha. I was just trying to figure out ways to have arms that move and maybe gesture, which could be done by giving say an arm different poses via the mouth shapes, but I bet it would change to fast.
Some others have asked for "parenting" or group rotating. I'll have to stew on it a bit to see if I can come up with a super simple interface for that.
Another idea is if I exposed the "smoothing" control. That way you could do hand gestures but have them move at a different speed than the mouth. The mouth-shapes do get "smoothed" so they aren't changing too rapidly, I may be able to make that a control that could be useful.
I'll add both of these to do the research list!
this is also something id be super interested in! specifically the "join a joint to another joint" part, I want to link the hand to eye tracking and have the forearm rotate with the shoulder and upper arm to give the appearance of drawing or using a mouse! im not sure if this would be difficult to impliment though..