Saving extracted objects from pictures using transparent bac

[JackRnl]

I know how I can extract an object from a picture using "Selection | set Shape | Polygon" followed by defining the polyon en Copy it.

After making such a selection and copying it onto the clipboard I CAN paste it into a picture and it functions as an overlay.
My problem is I don't know how to save that selection in such a way I can use it in the future as an overlay.
I GUESS I have to save it in a "transparent window" but don't know how to do that.

Any suggestions are welcome, thanks in advance

BTW I'm using 6.1.1. at the moment (without any plugins yet)