Retrieve Current iPhone Lock Screen Live Photo Wallpaper
I don't know of a way, but you can take a screen shot while in lock screen.

And if your unlocked screens have the same background, you can create a screen page with only one app on it, and take a screen shot of that.  In fact, when I tried it, I accidentally left the one app on its original page, so the page was empty.

Then use a graphic editor to combine the two, thereby removing much of the clock and the app icon.

I took the top half of the empty app page (no big clock) and the bottom half of the empty app page (no dock) to get the image below.

Then if you put the resulting image into reverse image search, you might get the original image.