If you are on Windows 10, the Aero theme gets installed at user creation by default. In the Aero theme it points to this location for the wallpaper:
C:\Windows\Web\Wallpaper\Windows\img0.jpg
To be really technically accurate the location is actually:
%SystemRoot%\web\wallpaper\Windows\img0.jpg
If you want to edit the Aero theme's default wallpaper you would mount Windows in NTLite and replace that image with anything you want, so long as it has the exact same name with the .jpg extension. Now, as a disclaimer I haven't personally done this myself, because instead I tweak Windows so that it uses no theme at all, but I have dealt with enough theme related stuff in W10 that I'm confident this is a solution. Let me know if it doesn't work though.
Any other approaches are more complex because it usually requires circumventing or editing the themes.