Read "The Diamond Age" by Neal Stephenson for one imagining of this scenario.

That's OK, I've been looking for reddit up-vote arrows this whole thread.

Live wallpapers are programs that write to an always-visible canvas, and thus need to be installed.

Some wallpaper apps keep their libraries online, and provide an easy-to-browse catalog of images. You only download the ones you want.

I may give it another shot some time in the future, but the last time I tried a debian install (on my work machine, so I didn't really care as much) the installer didn't even give me the option to choose my desktop environment, sticking me with gnome when I wanted KDE.

There is a reason for that. KDE was (is still?) considered non-free because of its use of the Qt toolkit. Debian is 'pure' in its Freedom, thus you have to install KDE from a non-free repository.