Yep, my students are always after me for this one. Here's a mock up I did for them.
<img src="http://www.thephotoforum.com/forum/attachment.php?attachmentid=53365"/>
Here's the two images with the Photoshop layer structure inset.
<img src="http://www.thephotoforum.com/forum/attachment.php?attachmentid=53363"/>
I dropped the Ryan photo on top of the general. I sized the Ryan photo down as it was too big -- lined up the eyes. A layer mask on the Ryan layer let me paint away Ryan's hair and suit. I did him a huge favor and gave him the general's ears. Then I copied the backgroud layer and placed that on top of the Ryan layer. I did this because I wanted to give Ryan the general's mouth but needed to move the mouth over a tad. So there's a mask on the top layer that just reveals the general's mouth.
I sampled the general's skin tone and applied that to Ryan's photo and then flattened the whole thing out and did a little healing brush work to smooth over some rough spots.
Joe