aspx当中加入
    <script type="text/javascript">
    function refreshimg()
    {
    document.getElementById(" <% =Image1.ClientID %>").src="getimg.aspx"; //image1为你的验证图片名.getimg.aspx是你产生图生的网页.
    }
    </script>

<span style="cursor:hand"> <asp:Image ID="Image1" runat="server" Height="16px" ImageUrl="getimg.aspx" Width="34px" /> </span>  //span里为改变鼠标为手型.

cs当中
    protected void Page_Load(object sender, EventArgs e)
    {
      Image1.Attributes.Add("onmouseup", "javascript:refreshimg();");
     
    }