Ok, the goal of the earlier Serenity project was to facilitate this one. I've made a triptych of images from Serenity:
I'm not entirely satisfied with the frames chosen around the actors cheeks in the third image, so I may end up doing some more work on this. This isn't really the final stage of the project - so the component tile images won't be recognizable on the web page. Still, I think it's pretty nifty. I'm fairly satisfied that the large areas of constant color (e.g. the white background behind River) are coming out with less annoying repetition. That's the result of two small fixes: 1) rejecting tiles that are already in use nearby, and 2) tiling the images in random order.
Larger images can be found at http://www.lecompte.org/Dave/Mosaic/Serenity/index.html