====== wallpaper ======

   ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml

Change 

xwallpaper. You should be able to install it with sudo apt install xwallpaper.

You can use command bellow to set your wallpaper*

xwallpaper --zoom path/to/your/wallpaper.jpg

et ...
xfconf-query -c xfce4-desktop -m
...and then change the background using the Settings Manager > Desktop.

The command monitors channel xfce4-desktop for changes. It will tell which property on channel xfce4-desktop is changed.

Then the command to change that property would be like this

 xfconf-query -c xfce4-desktop -p insert_property_here -s path/image