procedure TForm1.Button1Click(Sender: TObject);
 var
  b1, b2: TBitmap;
  c1, c2: PByte;
  x, y, i,
  different: Integer; // Counter for different pixels
begin
  b1 := Image1.Picture.Bitmap;
  b2 := Image2.Picture.Bitmap;
  Assert(b1.PixelFormat = b2.PixelFormat); // they have to be equal
 different := 0;
  for y := 0 to b1.Height - 1 do
  begin
  c1 := b1.Scanline[y];
  c2 := b2.Scanline[y];
  for x := 0 to b1.Width - 1 do
  for i := 0 to BytesPerPixel - 1 do // 1, to 4, dep. on pixelformat
  begin
  Inc(different, Integer(c1^ <> c2^));
  Inc(c1);
  Inc(c2);
  end;
  end;
 end;