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.