XFCE 命令 改变壁纸

An alternative method you can try.
Run in terminal:
xfconf-query -c xfce4-desktop -m

With the terminal open, change the wallpaper of the desktop using right-click on desktop > Desktop Settings…

See the output of changing the wallpaper in the terminal (for me it was /backdrop/screen0/monitor0/workspace0/last-image).

Copy the output and replace it in the following command:

xfconf-query -c xfce4-desktop \
-p /backdrop/screen0/monitor0/workspace0/last-image \
-s "/path_to_the_image/<name_of_the_image>.jpg"

Finally, change the path to the image in the last command and test it. If it’s working, you can use it in your script.

Source:
https://forum.xfce.org/viewtopic.php?id=10894 12