It's not the CSS source code, it's the HTML.
You have to change the html of your page.
Code:
<html>
<head>
<style type="text/css">
/*this is my style, but it has nothing to do with changing the image*/
</style>
</head>
<body>
<img src="http://theeradio.com/wp-content/uploads/2007/03/logo-header3.png" onmouseover="this.src = 'http://www.theeradio.com/wp-includes/images/logo%20header4.png';" onmouseout="this.src = 'http://theeradio.com/wp-content/uploads/2007/03/logo-header3.png'">
</body>
</html>
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