---------Start

   <script type = "text/javascript">
       
        function MyFunc()
        {
            if(document.getElementById("Image1").style.visibility == "" || document.getElementById("Image1").style.visibility == "visible")
            {
                document.getElementById("Image1").style.visibility = "hidden";
                document.getElementById("Image2").style.visibility = "visible";
            }
            else if(document.getElementById("Image2").style.visibility == "visible")
            {   
                document.getElementById("Image2").style.visibility = "hidden";
                document.getElementById("Image1").style.visibility = "visible";
            }
        }
         
    </script>


------------------------
<asp:Image ID="Image2" runat="server" onmouseover = "MyFunc();" ImageUrl="~/index.jpg" />
-----------------------------


------------End