Windows 10 does download automatically amazing pictures for the login and lock screen, but Microsoft wants you to spend money on themes so they've hidden the location of these files. They also frequently remove and replace them.
This script looks for HD (1920x1080) pictures within Windows' "hidden" download folder and copies them over to the user's Home in a configurable folder (by default "Pictures\Saved Pictures")
If needed, edit the $image_dest folder in the configuration section of the script
Then right-click on GetWindowsWallpapers.ps1 and choose Run with Powershell
Simply point your wallpaper Slideshow to this folder from your Personalization options in the Settings menu and enjoy your beautiful HD wallpapers!
I recommend creating a scheduled task that will run this copy upon every system start up with the line
powershell.exe -file [location of the script]\GetWallpapers.ps1
That's very possible. It's the first time I ever code anything in Powershell. So there are good chances it's horrible and full of bad practice. But it does the job.