Fancybox Code Change

Hi:
I am using fancybox to display images but want to alter some settings, namely having fancybox show one thumbnail but have multiple images show once the lightbox opens. I have found the code here – https://jsfiddle.net/uZCC6/ but am confused either where to place these files or which ones to edit. I understand the HTML portion.

The window shows a .js section and a .css section.

Can anyone offer any assistance?

Thanks