function InvertBitmap(Bmp: TBitmap): TBitmap;
var
 x, y: integer;
 ByteArray: PByteArray;
begin
 Bmp.PixelFormat := pf24Bit;
 for y := 0 to Bmp.Height - 1 do
 begin
  ByteArray := Bmp.ScanLine[y];
  for x := 0 to Bmp.Width * 3 - 1 do
  begin
  ByteArray[x] := 255 - ByteArray[x];
  end;
 end;
 Result := Bmp;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
 Image1.Picture.Bitmap := InvertBitmap(Image1.Picture.Bitmap);
end;