I think I have finally fixed it, for my setup at least...
I have left the "1 National Geographic" folder in Default Collections in place, but I have deleted the Nat Geo photos within that folder and replaced them with my own set of pix. With each phase of deleting the Nat Geo pix and replacing them with my own selection, the computer prompted me to enter my admin password.
I tried all the other suggestions in this thread, but invariably the computer would default back to the Nat Geo set.
It now still defaults back to Nat Geo, but by replacing the contents of that folder, I think I have an acceptable "workaround" here. Hope this helps.