You create multiple seeds of images, say in Photoshop, then you import them, create a big movie clip, create a movie clip for each one of them (the more you have the better it will look) place them nicely. Now the most important - YOU DEFINE THE ALPHA for each, making it look like one is beneath the other, then (I hope you have placed each in a diff layer) you animate their motion. Done.
--Play with it, it's fun--
--Mmmm...Sacrelicious! [Homa S.]
--Mmmm...Forbidden donut [Homa again]
--Blame the guy who doesn't speak Engish [ you can guess]
--Just because I don't care doesn't mean I don't understand.