You can change the large photo that appears on the page by either clicking on one of the small thumbnail photos, or by just passing the mouse over these thumbnail photos.

You decide the behaviour by either having "Preview Images: on", or "Preview Images:off". If you have "Preview Images: on" then the large picture will change just by passing the mouse cursor over the thumbnail. If you have "Preview Images: off" then you have to click on the thumbnail photo before the large picture will change.

You have the choice of changing either behaviour simply by clicking on the box labeled Preview Images: "on" or "off"