[sde]

make a new function that disables all the buttons, .. then call that function on click.

some suedo code:

private void DisableInterface()
{
  btnSend.value = "Please Wait...";
  btnSend.disabled = true;
  txtMyField.disabled = true;
  txtMyOtherField.disabled = true;
}

 private void Page_Load(object sender, System.EventArgs e)
{
  this.Image1.Attributes.Add("onclick", "DisableInterface();" + this.GetPostBackEventReference(this.Image1));
}

like i said, it's suedo code, .. but you get the idea.