Wouldn't they take up a lot of memory, though? I'm not sure how these things work.
I had a Star Trek screen saver for one of my early Apples that was pretty cool - it was fairly well animated, and oftentimes very full of activity, but the images were quite small.
I'm not that level of programmer, and I haven't the faintest idea of where one might get the basic bones of a screen saver programme, but I suppose it could be done...
-TD, interest suddenly piqued