I'm using Ubuntu 12.04 . If more information is needed, please tell me. I Tried some guides to cature my login screen:

How can I take a screenshot of the login screen?

http://www.howtoforge.com/how-to-take-a-screenshot-of-your-login-screen

I tried the first one, but after running:

$ sudo bash /tmp/shot.sh >/tmp/shot.xwd

I'm getting this:

No protocol specified
No protocol specified    
xwd unable to open display ':0'

and i'm getting an xwd file in my tmp folder. running the other commands giving errors.

tried the second one, after running:

chvt 8; sleep 5; XAUTHORITY=/var/gdm/:0.Xauth DISPLAY=:0.0 import -window root /tmp/gdm-login-shot.png

I got:

No protocol specified
No protocol specified
import: unable to open X server ':0.0' @ error/import.c/ImportImageCommand/366 

Many Thanks for helping (:

EDIT to vine_user: here the output from the terminal-I cacked it from within the system while I'm looged in:

ubuntu@PrecisePangolin:~$ echo 'sleep 5; DISPLAY=:0 XAUTHORITY=/var/run/lightdm/root/$DISPLAYDISPLAY xwd -root' > /tmp/shot.sh
ubuntu@PrecisePangolin:~$ sudo bash /tmp/shot.sh >/tmp/shot.xwd
bash: /tmp/shot.xwd: Permission denied
ubuntu@PrecisePangolin:~$ sudo su
root@PrecisePangolin:/home/ubuntu#  sudo bash /tmp/shot.sh >/tmp/shot.xwd
No protocol specified
No protocol specified
xwd:  unable to open display ':0'
root@PrecisePangolin:/home/ubuntu#

EDIT 2!: HERE IT IS!:

just uploading-this uploadong site is better than other iuset to use (: :