I have no clue what happened, but the desktop background picture has
ceased to be displayed. I'm not even sure where to look to troubleshoot
the problem. The salient information is: OS is fully updated Testing,
MATE desktop environment. I have attempted to re-set the desktop
background via both system settings and via right-click->set desktop
background on the desktop to no effect. Thinking that probably the
issue was associated with my normal login (the only one on the system),
I created a new user from root terminal and logged in with newly created
skeleton home folder; the problem persisted with the new user, so it is
a system-wide issue. Past that, I have no clue. Any ideas/help will be
greatly appreciated. Thanks in advance.