In Windows 10 I found the Windows desktop background under C:\Users\[your user]\AppData\Local\Microsoft\Windows\BackgroundSlideshow\.
The default lock screen pictures can be found here: C:\Users\[your user]\AppData\Local\Microsoft\Windows\LockScreenSlideshow\.
1 Answer
If you are asking where lies the default wallpapers, it's here
C:\windows\web\wallpaper
