Lock Screen Picture Grabber
This is a utility to retrieve Windows 10 Spotlight images from the lock screen and save them in your Pictures folder. It opens the images in the hidden data folder and lets you pick which ones to save (you'll need to set the filename before clicking save for a selected image, and it's too dumb to ask you whether you want to overwrite an exsiting image).
It has not been tested on computers other than my own. The code project is a Visual Studio 2010 WPF project, but should work fine on newer versions.
A prebuilt executable is in the Release folder:
https://github.com/Xangis/LockScreenPictureGrabber/blob/master/Release/LockScreenPictureGrabber.exe
If you just want to try the app, get and run that file.
This software is released under the MIT license.
Development Status
This was a one-off app that I wrote in about an hour because I wanted to save a Windows 10 login screen that I liked. It has not been thoroughly tested and I am willing to accept pull requests if you have improvements to contribute.