This directory contains the 4k Wallpaper that is part of Windows 10. Windows does not honor a custom background set in Group Policy and a Custom Theme during Feature Updates (Upgrades). Unfortunately the only way to resolve this is to overwrite the Wallpaper that exists in this directory. Replace the contents of the Web directory with your custom Wallpaper. For my Enterprise, we use a solid blue color. Selecting a solid color prevents issues with stretching images.