Chroma Photo is a free and easy to use green screen photo software for all Mac users.
To get a feel of how the software works, just click on the green or blue background in the dog`s photo that appears in the middle of the screen. You should see the old background being replaced with a new one.
Try scrolling through the list of background thumbnails on the right hand side of the screen and repeat the above process.
You can import your own blue or green screen photos by clicking on the "Add" button on the left (under FOREGROUND) and apply the backgrounds bundled with the software.