On my Windows 10 system, I really like the images that Microsoft provides on my lock screen. And I wanted get a copy of one of the pictures there to make it a background for one of my other computers. Well interestingly Microsoft has these sort of HIDDEN without FILE name extensions so when using a TOOL like agent ransack looking for all image file types on the system you will find everything BUT those images.

Went looking on google and found a solution as linked here: https://www.laptopmag.com/articles/find-windows-10-lock-screen-pictures

Instead of just grabbing a copy of largest file sizes, I took a copy of all and placed it into a folder at C:\win_lock_images
I the launched command shell via CMD at Cortana
With command shell open I performed CD\. then CD win_lock_images
Now at C:\win_lock_images I performed a wildcard rename to all files at that location by running rename *.* *.jpg
Now all files there have the .jpg extension and they have association as JPG images and now can be viewed for what they are.
Select whatever images you like and save them in another folder and delete the ones you dont want at say C:\win_lock_images if you used the same folder NAMING as I did.

*Microsoft changes these images from time to time and some images I saw a while back are gone. Not sure why they hide them like this under no file name extension and buried in app data of a user profile. They have the feedback system on these images where they will ask on the lock screen if you like these images and would like to see more like what you see etc and answering to that you can get a whole new group of images. 

Image attached shows the long alpha_numeric image file names that Microsoft uses, and with thumbnail view I was able to see a sample of all and sort by file size largest to smallest in size. The image I want is the one in top left which is greater than 1MB in size.  Yea many of the lock screen images that Microsoft provides look beautiful (Windows 10 Spotlight I guess is what that FEATURE is called)

Lot of them are good to use as standard desktop wallpaper too.Made a copy of them elsewhere in case Microsoft wipes them out and replaces them with new ones. Wonders now if there is some location at Microsoft where you can look at whats available to pick and chose. So far they have been a surprise as to what I will get next. Thanks for clarification on that its Windows 10 Spotlight for feature name.