SeeNow

I am surprised not many people have heard about this site. I'm using it a lot and never had a problem.

Upload a portrait and insert into hundreds of images. Face recognition technology gets the face centered for you, more or less, and then you can enlarge or reduce it and wiggle it around till it looks OK. Then you can either go back and see it already inserted into all the existing images, or you can OK it and post it to your blog or your choice of social networks or have it printed on a cup or a mousemat etc. Probably all been done before one way or another, but this is quick and easy and has everything there in one place. It's fun, for a while.

The downside for now is no image editing and the facial recognition doesn't extend to color matching. So the chances are that your picture is not going to match the color or texture of the image you're dropping it into. But if you're content with having your head on Arnold Schwarzennegger's body, you go for it. Personally I think it'd be funnier with my body under his head, but there you are.