function update_form(){
  var size = document.getElementById("image_size");
  var amount = document.getElementById("form_amount");

  switch( size.value ){
	case 'small': {
	  //document.select_image_form.
	  amount.value = 99;
	  break;
	}
	case 'medium': {
	  //document.select_image_form.
	  amount.value = 119;
	  break;
	}
	case 'large': {
	  //document.select_image_form.
	  amount.value = 139;
	  break;
	}
	default: {
	  //document.select_image_form.
	  amount.value = 0;
	  break;
	}
  }
  //alert(document.getElementById("form_amount").value);
  return (amount.value != 0) && (amount.value != null);
}