[ecfisa]

Hola.

Cambiá por ShowMessage, v.gr. :

Código Delphi [-]
procedure TForm1.FormCreate(Sender: TObject);
begin
  Image1.Picture.LoadFromFile(GetEnvironmentVariable('HOMEPATH')+'\Pictures\Blank.bmp');
end;

procedure CenterEllipse(Img: TImage; x1,y1,x2,y2: Integer; aColor: TColor);
var
  C,R: TPoint;
begin
  C.X := (Img.Picture.Width) div 2;
  C.Y := (Img.Picture.Height) div 2;
  R.X := (x2-x1) div 2 ;
  R.Y := (y2-y1) div 2;
  if (C.X-R.X < 0) or (C.Y-R.Y < 0) or
     (C.X+R.X > Img.Picture.Width) or (C.Y+R.Y >Img.Picture.Height) then
    ShowMessage('Elipse fuera de rango')
  else
  begin
    Img.Canvas.Pen.Color := aColor;
    Img.Canvas.Ellipse(C.X - R.X,  C.Y - R.Y, C.X + R.X, C.Y + R.Y);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  CenterEllipse(Image1, 17, 17, 328, 328, clRed);
end;

Saludos