Your theme takes advantage of WordPress’s built in galleries. Their display is provided by the Jetpack plugin. Enable the extra Tiled Galleries and Carousel by going to Jetpack > Settings. Once you’ve done that, you’ll notice you have a few new gallery types available when creating a gallery.
The gallery most used throughout the demo is Tiled Mosaic. This gallery generates a unique layout based on your image order and orientation. The effect is a nice, neat dynamic grid. Arranging your images can change the layout, so feel free to move your images around to get the layout you want. You can also choose to randomize your images to generate a unique layout upon each viewing.
If you enabled the Carousel feature in Jetpack, the images in your gallery will open in a lightbox for larger viewing.
To access the full features screen (as in the screenshot above), you must go to this url :
For an in-depth tutorial about the galleries, please visit the WordPress Codex.