Perhaps the single biggest cliché in the world of photo manipulation is "Photoshopping" one person's head on another person's body. Two points: First, just because it's a cliché, doesn't mean you can't have fun doing it. And second, you don't need Photoshop (or even a lot of photo editing skills) to pull this off. There are websites and apps that can do this for you pretty easily. Though, keep in mind that the results are often very goofy — you won't fool the CIA.
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There are a lot of websites and apps that do this on Windows and on mobile devices like the iPhone, but on the Mac desktop your options are more limited. Here are a few easy-to-use apps to try.
Face Swapper is a simple Mac app that promises to swap all the faces in a photo — and it works with photos that have many photos in the same image, from two and up. The downside? It's not free (though, it does only cost $2).
Print Creations for Mac
Here's a free alternative: Print Creations for Mac is an app that has a number of graphical tricks up its sleeve, among them the ability to swap faces onto other bodies in a photo. This program is template based and your options are limited, but it's free! A complete tutorial is located on the download page.