Also I noticed that anything other than worspace0 is not working from
emacs -nw, also from emacs -nw I have to extract monitor and screen
infos from command line (the emacs functions you sent are working only
on graphic display).
Here the last function I use here to test (works in emacs -nw and from
all workspaces) if that help.

(defun tv/change-xfce-background (file)
    (let* ((screen  (shell-command-to-string "echo -n $DISPLAY"))
           (monitor (shell-command-to-string
                     "echo -n $(xrandr | awk '/\\w* connected/ {print $1}')"))
           (desktop (and (display-graphic-p)
                         (x-window-property "_NET_CURRENT_DESKTOP" nil "CARDINAL" 0 nil t)))
           (prop    (format "/backdrop/screen%s/monitor%s/workspace%s/last-image"
                            (substring screen (1- (length screen)))
                            monitor
                            (or desktop 0)))
           (proc    (apply #'start-process "set background" nil "xfconf-query"
                           `("-c" "xfce4-desktop" "-p" ,prop "-s" ,file))))
      (set-process-sentinel
       proc (lambda (proc event)
              (if (string= event "finished\n")
                  (message "Background changed successfully to %s" (helm-basename file))
                (message "Failed to change background"))))))

Po Lu <luangruo@yahoo.com> writes:

> Thierry Volpiatto <thievol@posteo.net> writes:
>
>> Don't know where I can get that (/backdrop is not a directory).
>
> I meant with the `xfconf-query' program, sorry:
>
>   xfconf-query --list --recursive --channel xfce4-desktop --property /backdrop


-- 
Thierry