How do I modify a built-in screen saver source?

I want to use the same built-in screen saver Source (e.g., Cosmos, Ariel, National Geographic) for my screen saver, but I would like to add my own photos from the collection and maybe remove some photos. How do I do that?

For example, I figured out how to access the Cosmos.slideSaver and go into its package contents. I found the default photos are in the Resources folder. I added my own "Cosmos" photos to the Resources folder but none of my pictures showed up in the screen saver.

How do I do it?