This allows you to use the "print screen" key to take screenshots with scrot. The snippet above is to be placed in your openbox configuration file, inside the openbox_config and keyboard tags (with the rest of the keybinds). Since I use lxde, my openbox config file is in ~/.config/openbox/lxde-rc.xml.
You can use shift to copy your screenshots to your clipboard (requires xclip).
You can use ctrl to be able to select a rectangle or a window for the screenshot.
Scrot beeps when taking a screenshot. You can prevent this by adding -z in the scrot commands.