<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript">
function showimage1() {
document.getElementById("Image1").style.display = "inline";
document.getElementById("Image1").src = 'add_icon.png';
}
//you can write for other buttons as well
function hideimage() {
document.getElementById("Image1").style.display = "none";
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:Button ID="Button1" runat="server" Text="Button" onmouseover="showimage1();" onmouseout="hideimage()"/>
<asp:Image ID="Image1" runat="server" />
</form>
</body>
</html>