Regardless of what config says (’/usr/share/backrounds/warty-final-ubuntu.png’ )?
The file on lock screen is probably “Ubuntu-Mate-Cold-no-logo” or “Ubuntu-Mate-Radioactive-no-logo”. I will copy over my renamed picture…will report back soon.
EDIT:
It was “Ubuntu-Mate-Cold” and it solved my problem. Thank you very much Steve!!!