While there may be a way to capture the image you still can see on
the desktop, most of the methods I can think of won't save it as an
equally high resolution as an original; and perhaps not as large.
Depending on what source the image was brought into the computer
from, and if it was via a User account or put into a system file, an
image could be hard to locate; if the original is still in the computer.
And I have (out of curiosity) looked into my system stuff for an image
I've added to the Desktop as a background; while knowing a duplicate
or an original was available in another location, or a source folder.
I've noticed at different times, usually after performing some levels
of maintenance where certain caches and other items throughout
the System are affected, that a personally selected folder or image
file that had been accessed and available from the System Prefs
Desktop & Screensaver control panel, is no longer found. However
since I usually bring images into my computer by Date, from my
digital cameras, I can go back to locate them in at least one place.
And have not been successful in finding a System location for a
lead on where to look for the image, apparently so orphaned.
You could try to get a screen shot, for later sight comparison in
your other user folders. And also if you happen to make a clone
of your complete system for later access, since it may save the
image as a desktop background so long as it was present at a
point in time when the Clone was made.
I've made clones and found the same screen background to be
very distracting when booted into a familiar scene. So I change
them eventually (for reference that I've booted from a clone.)
Perhaps someone who has gone searching their folders & files
for the source of a Desktop image currently presented may be
of help in re-tracing steps to find where your latent image is in
hiding; since you can't find the source image you first loaded.
Sorry to be of little/no help.
Good luck & happy computing! 🙂