Photon is a great free image CDN that you can use with any self-hosted WordPress install via Automattic’s Jetpack suite of plugins. Photon uses wordpress.com’s infrastructure to host your site’s images on one of the fastest CDN globally.
I highly recommend enabling it on every WordPress install. If your site is on cheap shared hosting, it will dramatically improve page load times. If you’re hosting a huge news site, it’ll save you loads of money.
By default, Photon automagically serves any images embedded in or attached to a WordPress post or page. Including feature images, galleries, third-party sliders. Due to the nature of WordPress hooks and filters, it’s not possible for photon to grab images stored in post meta fields, or any images that are part of theme template files.
I’ve written a gist that exposes Photon’s CDN wrapper as a simple function you can call in templates: