Replace Lockscreen and Wallpaper in Windows 10

Rename your Background/Wallpaper you like to use for your deployment to Wallpaper.jpg and copy the Wallpaper.jpg to your MDT “scripts” folder.

Now add Powershell and .NET support to your WinPE image.

Then add an Command Line in the TaskSequence like this and copy the text below inside the Command Line like this.
powershell.exe -ExecutionPolicy “ByPass” -Command “Get-ChildItem \”%OSDISK%\Windows\img*.jpg\” -Recurse | % { Copy-Item \”%SCRIPTROOT%\Wallpaper.jpg\” $_.FullName -Force -Verbose }”

It is basically replacing all of the built-in Out of box wallpapers with my wallpaper.