OK, friends, by searching website again and again, I have found finally the solution,by doing different way, this problem can be solved. but still, I do not understand why original problem occurs or why original way does not work.
I appriciated it very much If any expert can give us some hints.
protected void Page_Load(object sender, EventArgs e)
{
this.ImageButton1.ImageUrl = image_url_default;
}