On mac OS there is a directory where you can put images in to replace default screens like the booting screen. Is there an equivalent for desktop enviroments like gnome, unity, kde, lxde, sfce etc.?
Ubuntu themes are available in the
All default backgrounds are in
However the boot splash screen is not part of the usual themes and managed by low-level components (plymouth) and unless you know what you're doing it's often discouraged to change the splash screen picture.