Hey, I've been wondering about this, always seems to be one of those hidden "Windows-type" features, sort of stored away somewhere secret. Anyways, looking into it in "The OS X Missing Manual" didn't offer much help, beyond using iMovie and designating an album of pics as a saver. No way to just drag and drop a custom movie (say, company logo, reel, whatever) into a folder somewhere.
So did some Googling and I found this little utility called iScreensaver Designer, and I wonder if anyone has tried this and what quick-tips you could offer. The Quick start guide seems pretty clear, but I'm not seeing a pointer to what size movies to use, I tried a 640x360 movie and it seemed to choke on it. It was 27 megs. It does say swf files are preferred, guess I'll make one and try it, but just wondering about custom screen savers in general, any wisdom would be great!
And in the search bar type "Screen Saver." The very first one that comes up does something sort of like this. Check that out. But then look at the rest. I'll bet there are others and you might find something else even closer to what you want.