Mate Desktop: Different Wallpaper on Multiple Monitors (HydraPaper)

The default Mate desktop only support single wallpaper.

You need to install HydraPaper.

Install Flatpak

sudo apt install flatpak

The Flatpak plugin for the Software app makes it possible to install apps without needing the command line.

sudo apt install gnome-software-plugin-flatpak

Add the Flathub repository

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo

Install HydraPaper

flatpak --user install -y flathub org.gabmus.hydrapaper

Run HydraPaper

flatpak run org.gabmus.hydrapaper

NOTE: It should appear under Menu -> Accessories -> HydraPaper. If it is no there, a restart might be required.

You might see the following message. You can fix this by performing a restart.

Note that the directories 

'/var/lib/flatpak/exports/share'
'/home/luaz/.local/share/flatpak/exports/share'

are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.

HydraPaper

To import photos from directory, click on the folder icon on top-left corner.

To randomly rotate background at interval, refer to HydraPaper Randomly Switch Background at Interval.