Take a screenshot of X display from SSH

Question

I'm trying to take a screenshot of a particular X display with imagemagick using the following syntax :

import -window root -geometry 640:480 -display :9999 myfile01.pcx

So taking the screenshot of root at display :999, but i get a :

import: unable to open X server `:9999' @ import.c/ImportImageCommand/367.

I'm logged in with SSH, is it possible ? what am i doing wrong ?

To be more precise, it's not really X, it's a Vnc server (launched by :

usr/local/bin/Xvnc :9999 -desktop client:9999 (9999) -auth /home/9999/.Xauthority -geometry 1024x768 -depth 24 -rfbwait 30000 -rfbauth /home/9999/.vnc/passwd -rfbport 9901 -fp /usr/share/fonts/X11/misc -pn -rfbauth /home/9999/.vnc/passwd -rfbport 9999 -alwaysshared +kb -desktop CyberUser[9999]