Some of these are believable, others look like cardboard cutouts, but nonetheless you'll likely browse through most of these photos with celebs 'shopped into Everett Hiller's holiday party for the sheer joy of spotting the doctored face. A little time and creativity, and those flash-blown photos take on a whole new life! How long before there's an app for this?
I start by finding photos that at least match the basic lighting conditions such as a flash used and general picture quality. Then I mask out the layer to my liking. Then I use the Replace Color tool under Adjust Image to match the color. I try to match the highlight tone and the shadow tones. If I absolutely need to then I will use the match color tool, but I don't usually have much luck with it.
Check out the full gallery over at Imgur.