Update Progress Gif Image Can't Be Displayed When I Click On A Button In Modal Pop Extender

I have an issue that I can't display my update progress GIF image when I click button in Modal Pop Extender the Aspx Code is : Copy

On the other hand, I added that code block on the Modal Pop Extender Button:

protectedvoidButton1_Click(object sender, EventArgs e)
{
    string script ="document.getElementById(";
    script += "'";
    script+=Panel1.ClientID;
    script += "'";
    script+=").style.display='none';";
    ScriptManager.RegisterStartupScript(this, this.GetType(), "A",script , true);
    System.Threading.Thread.Sleep(2000);

}

Many thanks to "Hitesh Sharma" and his reply to a similar issue through that useful link : http://forums.asp.net/t/1552551.aspx