{these functions assume BitMap : TBitMap 
 is a data member of Form1;
 a similiar function can be used to rotate 
 an image 180° or 270°}

procedure TForm1.Rotate90;
var
  x, y : cardinal;
  BitMap2 : TBitMap;
begin
  BitMap2 := TBitMap.Create;
  with BitMap do begin
    BitMap2.Width := Height;
    BitMap2.Height := Width;

    for y := 0 to Height - 1 do
      for x := 0 to Width - 1 do
        BitMap2.Canvas.Pixels[y, Width - x - 1] := Canvas.Pixels[x, y];

    Assign (BitMap2);
  end;//with...
end;

{this procedure basically swaps each pixel 
 on the left half of the BitMap with a correspond 
 pixel on the right half; a similiar function 
 could flip vertically}
procedure TForm1.FlipHorz;
var
  x, y : cardinal;
  HColor : TColor;
begin
  with BitMap do
    for y := 0 to Height - 1 do
      for x := 0 to Width div 2 do begin
        HColor := Canvas.Pixels[x, y];
        Canvas.Pixels[x, y] := Canvas.Pixels[Width - x - 1, y];
        Canvas.Pixels[Width - x - 1, y ] := HColor;
      end;
end;