Yet, for those wanting it to be installable from the PureOS Store: Seek out for “Gnome Screenshot”.
I’d like it better to have to functionality available by pressing some button(s) - but nonetheless.
Give Delay in Seconds of 3 and press big blue “Take Screenshot” Button, switch to the screen you want to capture and await the click-sound.
Great article on this from @Kyle_Rankin: https://puri.sm/posts/easy-librem-5-app-development-take-a-screenshot/