masking...
well, if you want to use masking, there are also different masking methods. the easiest way would be to open both images in photoshop, to make it a lot easier on yourself, you should probably use 2 images that are similar at least in the angle of the face and lighting, and maybe skin tone would help too, depending on the subject. also possibly image size/resolution, depending on what you want to do with the image.
with both images open, drag the image whose face you want to change onto the images window in photoshop, it will create a new layer on top of the image whose face you wish to use. with the layer that you drag and dropped selected, drop its opacity to a reasonable amount so you can try to line up where you want the head, the eyes and mouth are the best thing to use when trying to align them. with this layer selected, in the layers palette, click the 'add layer mask' button on the very bottom of the palette, its the second button from the left. now on that layer, theres a white box next to the thumbnail of that layer, click on that box, now the layer mask is selected, 2 important things to remember going in is that 1. black takes away, 2. whites adds it back. ok, with the layer mask selected, select the brush tool, make the foreground color black, and the background color white, so they are available as you need them, wit black selected as the foreground color, start painting what you want to replace in the image, you should probably start with the flow, opacity, hardness, etc at default, or 100%. you can change this to get different transitions between subjects as you need to, wow its too early for this, if you make a mistake, use white to undo it, just hit x to switch between the background and foreground colors, painting with white will add back what you've removed using black, around the edges of the head or areas where the image will merge, a softer edged brush may be more useful, shift+[ decrease the brush hardness for a wet/feathery edged brush,so what this mask is basically doing is allowing you to see through to the image behind and make changes and such, I need some coffee :biohazard , there are other steps I didn't mention above that may have made things easier, but I just woke up and started typing. you should be able to go from there, it's fairly simple to do. I should make a tutorial on this or something. o, don't forget, you can also use the burn tool to make the skin a little darker or the dodge tool to make it lighter if you need to, but don't go nuts. you can also change brush opacity when painting near edges for a smoother blending of images. ok, I'm off today, so I am going back to sleep, I'll check back after a while.