You can. I picked up Wallpaper Engine when it was on sale on Steam, probably like $5 or less. It has tons of already made animated scenes, and in theory you can make your own if you're into trying to figure that shit out (I'm not).
The best part about buying shit like that through Steam instead of other methods (including direct from the manufacturer) is that any computer you install Steam on can use it. I don't know if this one has a device limit, but many do. It doesn't matter one bit if you install it through Steam. I currently have it, and another program called DisplayFusion, on 4 different computers, one of which is this one at work. Not once has it ever told me I can't have them all running at once.
Shit, just looked, and normal price is only $3.99. It's like you almost HAVE to buy it for that cheap!