The most critical feature for me would be the ability to move (animate) an image along a path.   

At a minimum, being able to set a move in/move out path other than straight. 

But ideally, for example, if there is a garden scene with multiple images (bench, flowers, sun).  Being able to bounce a bunny across the scene or fly a bee through the air would be great. 

I can get around this now, in a complicated way, with an animated gif, but that's cumbersome.