Use mtpaint to grab a screenshot

And make it an application you can add to lubuntu panel.
Create the following file /usr/share/applications/mtpaint-grab.desktop containing :

[Desktop Entry]
Encoding=UTF-8
Name=Grab screenshot
Exec=mtpaint -s
Icon=camera
Type=Application
Terminal=false
Categories=Application;Graphics;

You can now add it to the Application Launch Bar in the Panel.

Reference

Ubuntu Forums post : http://ubuntuforums.org/showthread.php?t=1521606&p=9552908#post9552908