Simpsons family

So I was looking around on the internet, and got a good idea for a flash application for the website. I realized that the Simpsons have the same amount of family members as I do, as well as the same sexes of the kids (2 girls and 1 boy). I have below a picutre of the Simpsons that I altered with the threshold. Then on the website I'm going to have the color pictures of each of their heads individually pop up over the black and white, and then go back to black and white. Each character will do this separately. I am having trouble with the photoshop on windows though. I cannot figure out how to put one picture on top of another. Can you try to tell me how to do it? Otherwise I like this idea a lot, and am going to try and work on it more in class because the computers are easier to use there. But here is the threshold picture atleast.