eBay and a lot of other sites create a new thumbnail image from the larger one, usually. However, you can try this:
<img src="image.jpg" height="250" width="120" onclick="window.open('http://www.website.com/image.jpg', '_blank', 'width=800, height=500');" />
In the part, "window.open......", the URL should be the same as the image's source.