This is how I setup lxde (on Fedora 14) to take screenshots.

• Install the gnome-utils package. This contains the gnome-screenshot program. It’s possible to avoid this by using imagemagick or scrot, but I couldn’t find a way to make them take shots of the active window (the usual alt+printscreen behaviour)
• Insert the lines below into the keybind section of ~/.config/openbox/lxde-rc.xml


    <keybind key="Print">
      <action name="Execute">
        <command>gnome-screenshot</command>
      </action>
    </keybind>
    <keybind key="A-Print">
      <action name="Execute">
        <command>gnome-screenshot -w </command>
      </action>
    </keybind>

Simples