if AdvTaskDialog1.Execute = mrYes then
  begin
    if bChecked then
    begin
      { Remove the item from the collection }
      Collection1.DeleteImage(idx);
      Collection1.Update;
    end;
  end
  else 
  begin
    { uncheck the checkbox if the image is not deleted }
    bChecked := False;
  end;

procedure TForm1.Collection1CheckboxClick(Sender: TObject; idx: Integer;
  var bChecked: Boolean);
{ Collection1CheckboxClick. }
begin
  { Delete the checked image? }
  AdvTaskDialog1.Clear;
  AdvTaskDialog1.Title := 'Warning';
  AdvTaskDialog1.Instruction := 'Delete This Image';
  AdvTaskDialog1.Content := 'Delete this image from the collection?';
  AdvTaskDialog1.DialogPosition := dpOwnerFormCenter;
  AdvTaskDialog1.Icon := tiQuestion;
  AdvTaskDialog1.CommonButtons := [cbYes, cbNo];
  if AdvTaskDialog1.Execute = mrYes then
  begin
    if bChecked then
    begin
      { Remove the item from the collection }
      Collection1.DeleteImage(idx);
      Collection1.Update;
    end;
  end
  else 
  begin
    { uncheck the checkbox if the image is not deleted }
    Collection1.Checked[idx] := False;
    Collection1.UpdateImage(idx);
    Collection1.Update;
  end;
end;