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e Canvas.Pixels for changing your image to image as mirror.

procedure TForm1.Button1Click(Sender: TObject);
var
  Bitmap: TBitmap;
  i, j: Integer;
begin
  if OpenDialog1.Execute then
    Image1.Picture.LoadFromFile(OpenDialog1.FileName);
  Bitmap:=TBitmap.Create;
  with Image1 do
  begin
    Bitmap.Height:=Height;
    Bitmap.Width:=Width;
    for i:=0 to Width-1 do
      for j:=0 to Height-1 do
        Bitmap.Canvas.Pixels[Width-i, j]:=Image1.Canvas.Pixels[i,j];
    Image2.Picture.Assign(Bitmap);
  end;
end;