Open image.

Using the Magnetic Lasso, select the face.

Open the image you wish to apply and drag it onto the face.

Chose Layer>New>Layer via Copy

On the Layers Palette choose Overlay

You should have three layers.

With Layer 1 selected, use the Magic Wand to
select the area outside the face.

Here you see the area selected.

Select Layer 2 and hit the Delete key.

As you can see the area outside of the face has been deleted and you are left with the image overlay.
I guess you could also do this by dragging an image onto the face and using the Opacity command,
make the image more transparent and then "erase" the area outside the face.