function RgbToGray(RGBColor: TColor): TColor;
var
  Gray: byte;
begin
  Gray := Round((0.30 * GetRValue(RGBColor)) +
  (0.59 * GetGValue(RGBColor)) +
  (0.11 * GetBValue(RGBColor )));
  Result := RGB(Gray, Gray, Gray);
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  h, w: integer;
begin
  for w := 0 to Image1.Width-1 do
    for h := 0 to Image1.Height-1 do
      Image2.Canvas.Pixels[w, h] :=
      RgbToGray(Image2.Canvas.Pixels[w, h]);
end;