Dear Maintainer,

         There seems to be an issue generating thumbnails in
mate-appearance-
properties, namely when selecting a wallpaper, I'm greeted with a
question mark
inside a triangle icon.

    * What led up to the situation?
         I have installed debian 11 alpha 3 amd64 netinstall today (Dec
8th
2020) at 03AM EST. After logging in, installing firmware-amd-graphics
for
         full resolution and rebooting *once* I was beginning to
customize my
desktop. I copied my folder of usual wallpapers, and opened
mate-control-
center's mate-appearance-properties
         to change the wallpaper.
    * What exactly did you do (or not do) that was effective (or
      ineffective)?
         I tried double checking the permissions on all wallpapers (they
*can*
be applied, just not thumbnailable
         I googled the issue and attempted to remove ~/.cache/thumbnails
and
relog. to no avail
    * What was the outcome of this action?
         Selecting any of the wallpapers I have (jpg/png formats) I get a
<?>
icon on the right side of the selection window
    * What outcome did you expect instead?
         I expect to see a thumbnail of the wallpaper selected