In reply to Alec Kinnear.

Jetpack's Galleries require a third-party connection (WordPress.com) because the mosaic display (highlighted in this review) dynamically determines the image size best for the display. At this time, WordPress doesn't have a way to dynamically generate thumbnails—only what is preset via Settings->Media or via the theme's code, which doesn't automatically regenerate older images if the sizes change.

To get around this, Jetpack loads the gallery images through our Photon CDN ( https://developer.wordpress.com/photon ) which includes dynamic image sizing as a feature.

The Photon code, though, is open-source so someone could run their own instance of Photon and use Jetpack's code to do the same thing without third-party connections. Available via SVN at http://code.svn.wordpress.org/photon/