procedure InvertBitmap(Bitmap: TBitmap);
type
    TRGB = record
      B, G, R: Byte;
    end;
    pRGB = ^TRGB;
var
    x, y: Integer;
    Dest: pRGB;
begin
    Bitmap.PixelFormat := pf24Bit;
    for y := 0 to Bitmap.Height - 1 do
    begin
      Dest := Bitmap.ScanLine[y];
      for x := 0 to Bitmap.Width - 1 do
      begin
        with Dest^ do
        begin
          R := 255 - R;
          G := 255 - G;
          B := 255 - B;
        end;
        Inc(Dest);
      end;
    end;

    form1.Image1.Picture.Bitmap.Assign(Bitmap);
end;

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