If you have a thumbnail image that you want to show in a bigger window, a popup window can be a great solution. In this example there are two images. image1.jpg and image1_thumbnail.jpg.

<a href="images/image1.jpg" target="popup"
onClick="window.open('images/image1.jpg', 'popup', 'width=640,height=460,menubar=0,toolbar=0,status=0,
scrollbars=1,resizable=1,location=0'); return false">
< img src="images/image1_thumbnail.jpg" alt="Image 1" border="0"></a>