How to take screenshot of login screen

Step 1:
Make /tmp/shot.sh
echo 'sleep 20; DISPLAY=:0 XAUTHORITY=/var/run/lightdm/root/$DISPLAY xwd -root' > /tmp/shot.sh

Step 2:
Login via text mode
Type CTRL+ALT+F1

Step 3:
Execute command
sudo bash /tmp/shot.sh >/tmp/shot.xwd

Step 4:
Return to GUI
Type CTRL+ALT+F7

Step 5:
Wait a few seconds, then log back in.

Step 6:
Login by normal way.
Open terminal and type:
convert /tmp/shot.xwd /tmp/shot.png

Reference:
http://askubuntu.com/questions/177067/capture-login-screen-not-sucssecful
http://ptspts.blogspot.jp/2010/02/how-to-create-screen-shot-of-gdm-login.html