Where are Apple's default desktop images stored?
They're in /Library/Desktop Pictures/
As of macOS 10.15 (Catalina) the location is /System/Library/Desktop Pictures. I'm unsure what version the location switched.
As of macOS 12:
- Desktop JPEG wallpaper are located in
/Library/Desktop Pictures - Pre-installed dynamic wallpapers (HEIF) are located in
/System/Library/Desktop Pictures - Any dynamic wallpapers downloaded by the user (from the "Desktop & Screen Saver" preferences) are located in
/Users/$USER/Library/Application Support/com.apple.mobileAssetDesktop