You could try something like the following:
- Open both files in GIMP.
- In the image you want to extract the face from, select the hand-drawn selection (lasso) tool, ticking the anti-alias and feathering boxes, with a suitable feathering radius so that the edge of the selection gets faded out instead of being hard-edged.
- Zoom in and carefully draw around the face until it is fully selected.
- Press Ctrl-C to copy the selection into the clipboard.
- Select the destination image.
- Press Ctrl-V to paste the face onto the image.
- Drag the selection to position the face precisely.
- Click outside the selection to anchor the face to the image.
Of course it might not be that simple. If the faces are different sizes or shapes, it gets more complicated.