This default fallback image can be changed by editing the group policy setting Computer Configuration > Administrative Templates > Control Panel > Personalization > Force a specific default lock screen image, although this setting only affects Windows 10 Education and Windows 10 Enterprise. If the Content Delivery Manager has no new picture to offer after you dislike the current one, Windows Spotlight falls back to a default static image. The current spotlight image can be found at C:\ProgramData\Microsoft\Windows\SystemData\ \ReadOnly\LockScreen_O, while the current and all previous images can be found in %localappdata%\Packages\_cw5n1h2txyewy\LocalState\Assets. Windows Spotlight's Images are provided by Windows' Content Delivery Manager.
