If you have a set of related images that you would like to group, follow step one but additionally include a group name between square brackets in the rel attribute.
<a href="images/pets1.jpg" rel="lightbox[roadtrip]">image #1</a>
<a href="images/pets2.jpg" rel="lightbox[roadtrip]">image #2</a>
<a href="images/pets3.jpg" rel="lightbox[roadtrip]">image #3</a>
No limits to the number of image sets per page or how many images are allowed in each set. Go nuts!