I have Lubuntu 19.10. The default tool is
screengrab --help shows the available options:
-h, --help Displays this help.
-v, --version Displays version information.
-f, --fullscreen Take a fullscreen screenshot
-a, --active Take a screenshot of the active window
-r, --region Take a screenshot of a selection of the screen
-m, --minimized Run the application with a hidden main window
Instead of opening a terminal and running something like
sleep 5 && screengrab -r which will launch screengrab after 5s to allow you to minimize the terminal window and view the region you're interested, it is worth making a keyboard shortcut for the purpose.
To make this shortcut, click on Lubuntu's Menu icon (usually located at the left end of your panel) and then open Preferences > LXQt settings > Shortcut keys. I've chosen Ctrl+Meta+I (where Meta is the Windows or Super key and I is for interactive.
Setting a shortcut for taking a screenshot of *a selected area
Once you save the shortcut, if you press Ctrl+Meta+I, you'll see this which is self-explanatory: