<public:attach event="onmouseover" onevent="btn_change1()" />
<public:attach event="onmouseout" onevent="btn_change2()" />
<script language="javascript">

function btn_change1()
{
aaa=this.style.backgroundImage;
this.style.backgroundImage='url(images/btn_hover.gif)';
}

function btn_change2()
{
this.style.backgroundImage=aaa;
}
</script>