можно ещё проще
<script type="text/javascript">
function onBut(elem){
elem.src = "http://www.google.ru/images/logos/ps_logo2.png";
};
function outBut(elem){
elem.src = "http://img.yandex.net/i/www/logo.png";
};
</script>
<img height="100" src="http://img.yandex.net/i/www/logo.png" onmouseover="onBut(this)" onmouseout="outBut(this)"><br/>
<img height="100" src="http://img.yandex.net/i/www/logo.png" onmouseover="onBut(this)" onmouseout="outBut(this)"><br/>
<img height="100" src="http://img.yandex.net/i/www/logo.png" onmouseover="onBut(this)" onmouseout="outBut(this)"><br/>
<img height="100" src="http://img.yandex.net/i/www/logo.png" onmouseover="onBut(this)" onmouseout="outBut(this)"><br/>
<img height="100" src="http://img.yandex.net/i/www/logo.png" onmouseover="onBut(this)" onmouseout="outBut(this)"><br/>