If you want to change something in the way the magnifier works, you'll need to include a config file too in your header:
<script type="text/javascript" src="tjpzoom.js"></script>
<script type="text/javascript" src="tjpzoom_config_default.js"></script>
</head>
I've prepared some config files for you to try (tjpzoom_config_*.js). Why don't you try what they do?
*
Notice the difference?