well, my gimp is german so I hope I say the correct technical terms of the tools in GIMP.
Try the "free selection" tool, which looks like a lasso.
just go roughly along the heads and close your selection by reconecting to where your selection started (sounds strange, sry)
then copy it (strg+c) and put it in the background (strg+v).
Gimp automatically creates a new level for your copied head.
now you zoom in and erase all the pixels that dont belong to the head.
afterwards you can move it wherever you want.
I hope this helped, but I guess it sounds rather confusing