Swapping image within selection

[Blighty]

I don't quite understand what you are trying to achieve. I'm also not to sure what you mean by a mask (Gimp has layer masks). Perhaps you could attach a screenshot showing all your layers etc.

Is the attached file what you are after? You can move the 2 lower layers around with Move tool.
(xcfgz is a compressed xcf file - use File > Open to open it)