In the Lunos Debug menu (hit insert), type “hud” to hide the hud.
F9 takes a regular screenshot.
In the Unreal console (~), type “HighResShot 2” to take a double res shot. Sometimes looks weird, can use other numbers (can be super slow!)
Screenshots end up in \LunosProject\Saved\Screenshots\WindowsEditor. All this should work in a cooked build as well.