Where, or what format, are the images used by Windows 10 when logged out or in lock-screen?
I searched the entire drive for *.jpg, *.bmp, *.png, and *.tif but the one I was looking for did not appear.
At first glance, Path to current desktop backgrounds in Windows 10?, although not exactly a duplicate, appeared to have the path I wanted. But when I went there, the pre-login image was there, but the lock-screen image was not. (And contrary to the claim in that answer, the files there did have extensions.)
UPDATE: I just did a lockscreen, and got an image that IS in that directory, but earlier today, it was consistently an image of an interesting footbridge that is not in that directory.