If I want to take a screenshot on Lubuntu, I can simply press PrtSc on my keyboard and have the file available in my home folder.

What I discovered recently is that there's a shortcut to take a screenshot of only the window: Alt + PrtSc.

Sadly, Shift + PrtSc doesn't work to take a screenshot of a selection of the screen, but I think it requires some additional editing on ~/.config/openbox/lubuntu-rc.xml.