On the Mac you can use Shift-Command (⌘)-3 (for whole desktop screen) or Shift-Command (⌘)-4 just for selected area. More detailed instructions here.
On the Windows you can use shortcut with Windows logo + PRTSCN (for whole desktop screen) or you can use Snipping Toolfor selected area. More detailed instructions here.
You can paste this copied image directly from the clipboard into the comment window in Freelux. And with known abbreviations: Mac CMD + V, Windows CTRL + V.
Among the embedded files you can easily write texts and describe pictures. This is useful if you want to explain the individual files or screens.