Chrome OS: https://storage.googleapis.com/chromeos-wallpaper-public/manifest_en-US.json and /usr/share/wallpapers
Fedora: https://download.fedoraproject.org/pub/fedora/linux/releases and https://archives.fedoraproject.org/pub/archive/fedora/linux/releases
Generic/elementary OS: /usr/share/backgrounds
iOS: /Library/Wallpapers
macOS: /Library/Desktop Pictures