Jetpack’s Photon module is indeed a free Image CDN:
https://jetpack.com/support/photon
It caches and serves all your images from our CDN. It’s a great start, and will help with your site’s performance, especially if you have a lot of images.
That said, caching plugins like WP Super Cache allow you to do even more, by using a CDN to serve all static resources on your site: images, but also CSS and JS files. Jetpack’s Photon module doesn’t touch those files.
If you’d like, you can use both Photon and another CDN to serve all static resources on your site; this way you’ll be using a CDN for all static resources on your site.
I hope this clarifies things a bit. Don’t hesitate to let me know if you have more questions!
