How do I change the display and functionality to match one of the samples given?

Using the Montauk template, my portfolio pages "stack" images one above the other. The style I want to see is on this page, where there is one main image and many tiny thumbnails below it. Users can either click on the main image to go to the next one or select from the tiny thumbnails:

http://www.squarespace.com/templates/?q=montauk%20portfolios - click on the Geoff Captain Studios example, then the "weddings" tab.

Is this done by CSS customization or is there a style manager button I missed?

Thanks!

What you're seeing on that sample site is not a gallery in an index, rather a gallery block on a page. Simply add a regular page, then add a gallery block and choose slideshow.

That's it. I just had to think about it differently. Thank you!