Faithful - Portfolio Page Setup

You'll need to have the Jetpack plugin installed, turned on, and then synced with your WordPress.com account (free account). 
Next go to Jetpack>Settings and Activate the Tiled Gallery option towards the bottom of the page.

Once that is activated, you will go edit your page and add a WP gallery to your page and from the Type drop down, you will select the Tiled Mosaic option.