I already tried both Shift+Ctrl+print and Shift+SupKey+print but nothing seems to happen. Already visited here Lubuntu keyboard shortcut for menu with no result.
Did you check your home directory? When I hit print screen (in Lubuntu 17.10) I get no indication that anything happened, but when I look in my home dir, the desktop picture is there.)
there may be a setting in LXDE but you could just bind any key to
~/.config/openbox/rc.xml (or possibly
lxde-rc.xml for lxde sessions)
/etc/xdg/openbox/rc.xml look for
<!-- Keybindings for running applications --> <keybind key="W-e"> <action name="Execute"> <command>scrot</command> </action> </keybind>