On Sun, 24 Jun 2018 08:19:02 +1000, in alt.comp.os.windows-10 you wrote:

I don't know how to find out the exact location

I posted all known locations of lock-screen spotlight photos over he
http://www.pcbanter.net/showpost.php?p=3759001&postcount=3

but you can often find
the general location by going to the Themes folder. The Themes folder
is located at:
\username\AppData\Local\Microsoft\Windows\Themes\
R-click an image and navigate to Properties and then Details.

Do others concur that this is related to the Spotlight lock-screen photos?

All I have in this location (1803 version) is a 2KB "custom.theme":
%localappdata%\Microsoft\Windows\Themes\custom.the me

When I doubleclick on that 2KB custom.theme, it appears to be simply a
widget which brings up the same user interface as if I had gone to
Start Settings Personalization Themes

While I never delved into lockscreen spotlight photos prior to this
question, I don't see any relationship (yet) with themes. Do you?

PC Experts - do you know how best to access this lockscreen location?
C:\ProgramData\Microsoft\Windows\SystemData\.