This plugin, like all of the other "lightbox" like plugins, operate based on links and not on images.
So basically what you are doing is:
<img src="http://example.org/image.jpg: />
But what you need to do is:
<a href="http://example.org/image.jpg" alt="image title"><img src="http://example.org/image.jpg: /></a>
This plugin will automatically add the activator rel attribute to the link. However, if you want to add the activator rel attribure yourself you can add rel="shadowbox" to the anchor tag like so:
<a href="http://example.org/image.jpg" rel="shadowbox" alt="image title"><img src="http://example.org/image.jpg: /></a>