Okay, this one is actually incredibly easy, but still, it was fun when I found out.
Windows 7 is by no means my favourite OS, but in my opinion it has some very “desktop themes”, packs of wallpapers which after install will rotate on your desktop. In Windows, it’s as simple as clicking “Download”, after which the pack will unzip itzelf and show up somewhere in your Appearance settings.
When I downloaded one in Arch, Ark recognised it as a zipfile, but didn’t recognise the .themepack extension. After some googling, I found out that the zip format is actually 7-zip, so simply renaming the extension to 7z does the trick (if you have p7zip installed, of course). KDE and GNOME can do rotating backgrounds too, so the rest should be easy.
Like I said, obvious really, but I liked it.