How to display preview of image before upload?

How to show preview of image?
I am convert image fiels into byte array values then store it in the session value. Access that session value in Default2.aspx value bind it as image in Image control. This image control shows preview now.

if (FileUpload1.HasFile)
{
  dt.Clear();
  postfile1 = pstFile(FileUpload1);
  Session["b"] = postfile1;
  dr = dt.NewRow();
  dt.Columns.Add("sname");
  dt.Columns.Add("simage",Type.GetType("System.Byte[]"));
  dr["sname"] = TextBox1.Text;
  dr["simage"] = postfile1;            
  dt.Rows.Add(dr);              
  Session["dt"]=dt;
  Image1.ImageUrl="Default2.aspx";
  Button1.Enabled = false;
  Button2.Enabled = true;
}

protected void Button2_Click(object sender, EventArgs e)
{        
 if (Session["b"] != null)
 {
   sqlcon.Open();
   query = "insert into stud(sname,simage) values(@sname,@simage)";
   SqlCommand cmd = new SqlCommand(query, sqlcon);
   cmd.Parameters.Add("@sname", SqlDbType.VarChar).Value = TextBox1.Text;
   cmd.Parameters.Add("@simage", SqlDbType.Image).Value = ((byte[])Session["b"]);
   cmd.CommandType = CommandType.Text;
   cmd.ExecuteNonQuery();
   sqlcon.Close();
   TextBox1.Text = "";
   Image1.ImageUrl = "";
   Label1.Text = "Image Uploaded Successfully";
   Button1.Enabled = true;
   Button2.Enabled = false;
   Session.Remove("b");
 }       
}