procedure TForm1.ImageEnView1DrawBackground(Sender: TObject; ACanvas: TCanvas; ARect: TRect; var Handled: Boolean);
begin
  Handled := true;

  // the whole background is green
  ACanvas.Brush.Color := clGreen;
  ACanvas.FillRect(ARect);

  // the image background is yellow
  ACanvas.Brush.Color := clYellow;
  with ImageEnView1 do
    ACanvas.FillRect(Rect(OffsetX, OffsetY, OffsetX + ExtentX, OffsetY + ExtentY));
end;