It's pretty easy. After you've put your zoom script into your header...
<script type="text/javascript" src="tjpzoom.js"></script>
</head>
... you slightly modify your <img> tag:
And now take a look at the source of this file. We've changed the TJPzoom() function to TJPzoomif(), which always checks a variable and only activates the zoom script when the value is 1. And we added a switcher, which changes that variable.