This tutorial explains how to create the image galleries used on my photo site.
This example contains 10 rather than 15 images, and the main image area is smaller, but clearly all the numbers can be changed as required. The relative position of the boxes is irrelevant, they can be moved around at will, for instance the thumbnails could run vertically down the page with the main image to their right.
This method does not permit drag-and-drop of images from the Finder or iPhoto. If using iPhoto then export or drag the required images to a folder in the Finder.
CCS Layout (layers) must be enabled.
Now repeat the process for the thumbnails, ensuring that the image imported into the thumbnail1 box is the same image that you imported into what was the image1 box, that thumbnail2 corresponds to image2 and so on, so that each thumbnail rollover triggers the correct image. Preview to check that everything works as it should.