Where is the picture saved after Win10 computer desktop wallpaper application is successful?

Under% AppData% \ Microsoft \ Windows \ Themes,
TranscodedWallpaper is a real wallpaper file, but there is no suffix format. In fact, Windows is converted to match the resolution of your monitor screen. You can copy it and change it to jpg.

There is a spare

% AppData% \ Microsoft \ Windows \ Themes \ CachedFiles

added, if you are referring to the wallpaper that comes with the operating system then under C: \ windows \ web