Wallpapers set from Photos app:
%LocalAppData%\Packages\Microsoft.Windows.Photos_8wekyb3d8bbwe\LocalState\PhotosAppBackground
Current wallpaper copy:
%APPDATA%\Microsoft\Windows\Themes\CachedFiles
%LOCALAPPDATA%\Microsoft\Windows\Themes\RoamedThemeFiles\DesktopBackground
Current wallpaper path:
HKCU\Control Panel\Desktop\WallPaper
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers\CurrentWallpaperPath
History of wallpapers:
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers\BackgroundHistoryPath0
HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers\BackgroundHistoryPath1
...
Registry keys are duplicated in HKEY_USERS\S-1-5-...
:
HKEY_USERS\S-1-5-...\Control Panel\Desktop\WallPaper
HKEY_USERS\S-1-5-...\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers\BackgroundHistoryPath0
...
Standard wallpapers:
%windir%\Web
%windir%\Web\Screen
...
Photo app cache (may contain thumbnails for wallpapers):
%LocalAppData%\Microsoft\Windows\INetCache\thumbnails\normal