Hi Everyone - Something I've not done before, so bear with me. This is what I need to do. On an existing video of two people at a party, I need to superimpose two heads from other people. The superimposed heads would be 'stills', on the moving bodies of the other two guests. I'd like it to have the still heads to follow the bodies around as they move around the party. I know this is possible - but how? I have iMovie. All help appreciated.