Try this..
<script language="javascript" type="text/javascript">
function mouseOver()
{
document.Button1.src ="login-button.gif"
}
function mouseOut()
{
document.Button1.src ="submit.jpg"
}
</script>
on button write this code
<asp:button id="Button1" runat="server" text="Button" onmouseover="mouseOver()" onmouseout="mouseOut()" xmlns:asp="#unknown">