By default LXDE doesn't have ability to take screenshot using printscreen. You need another application to capture the screen. I usually using scrot and gnome-screenshot.
Scrot is screencapture program that run from terminal/ command line.
gnome-screenshot is GUI screencapture software.
But i prefer gnome-screenshot for easy solution.
OK, install the apps.
#pacman -S scrot
Manual of scrot:
The results is png, place at home user.
#pacman -S gnome-screenshot
This apps will be installed and available on accessories menu, here's the main GUI:
To make gnome screenshot available when press Printscreen, you can edit .config/openbox/lxde-rc.xml
Looks like this picture:
Done, logout & login, then try press screenshot. You will see take-screenshot option.