While I was hunting around for some links to go with the character cards post, I came across the Morphases Editor, an online composite face generator. I put a link in comments, but this one is so interesting and potentially helpful in creating character portraits that I thought I'd put up a finished example, too, to show you all what can be done with it:
This is something like my vision of my character Will Scarlet. I took elements from five different faces and manipulated them to generate the composite, so it's my creation and does not require a model's release form for me to use the image. If I decided to use this in a promotional character card, it would need more work -- I'd like to run it through photoshop to clean up the image, change the length of his hair, even out his skin tone, pack up those dark bags under his eyes and otherwise fiddle with it -- but it's a very good place to start.