Please forgive me If this isn't the right place for my question, but after searching around a bit i find that I do not know what to search for! I can't think of what this effect would be called and have been trying to figure it out for about 2 weeks. All I want to do is create an effect wherein I replace the heads in a video with still images from pictures I have. This little flash video is pretty much what Im trying to figure out.

ZOMG ZUFALL! #1494

Im looking for someone to point me in the right direction, with maybe what to search for to find a tutorial or even directions. I know this is cheesy but I'm working on a video and need this for a joke. I do plan on using Sony Vegas if possible.