I tried it a few years ago.
Used the screen saver Aquarium (tropical fish swimming around).
You could do it via terminal. I forget how. It got too much after a few days.
Maybe a search here will find the terminal command.
vista (ultimate atleast) has dreamscene/scape which can access any video which has a codec installed. Works ok but wouldn't bother with it, uses too many resources.
If you want to make one of these, its just a case of making a move.
If you make up a sequence of images and then set the desktop to change every 5 seconds it creates a sort of moving desktop image. Ive seen it were someone did a jug slowly emptying.