Not to be rude... but if you edited the Firestarter background (for example), and had it load a random image from a directory on the server, then it would be possible. These flash backgrounds are simply images. Whether they are animated or static or moving is just how they are programmed. Some have a backgroud built-in or in resource folder (e.g. /theme/images/).
So surely this "would be possible"? Whether it is "recommended" is another story. This could dramatically slow down the loading of your site, and I would only use repeating images rather than large full-size / cropped photographs! This WOULD be a nice feature for the home and album listing pages
There are already a lot of scripts out there that display dozens of wallpapers and are continuously very very slowly scrolling horizontally.
I wonder whether this will be in X3. Still, I believe and feel other features in X3 would be worth being development over this feature, as nice as it would be.
Keep up the great work!