If you're going to be adding new people, places, or things to your photos, you'll want to add them as new layers. When something is on a layer, you can edit it separately from the rest of the image.

For example, if you've got a picture of a child and want to add his dog into it, paste the dog image into a new layer. Doing so allows you to resize doggy without changing the child's size. You can also erase a little around doggy's edges without erasing any of the background on the child's layer. Whatever you do to the dog's picture doesn't affect the child's image, and whatever you do to the child doesn't affect the dog.