Not that I have learned how to do it yet, but some files are easier to remove the background from. Then you can play with the image and change the background or combine images. RAW images are used for some of this. Andy P. is an expert in this. Ask him. By the way, RAW images take up more space (I believe) on digital cards. I am no expert by any means. I am still learning how to use my new digital camera.
A png is a file format (developed after jpeg,) that has some real good versitiliteis. The principle one (as I understand anyway) is the ability to cut out a portion of a pic, and save that portion without any background.
Once the portion of a pic is extracted from the original, it can then be added as a layer to another pic.
Most editing software has this capability. If you need more of an explination, or help, let me know.