Improve image quality with thumbnail

Colleagues

I'm adjusting a system where some images already exist in a certain directory, the thumbnail handling of the files was not done and the images were taking a while to open, since they are using the bootstrap thumbnail.

<img src=\"produtos/".$foto."\" data-imagezoom=\"true\" class=\"img-responsive thumb-image\">

To reduce the impact, I created a method where I decrease the image with php:

public function gerarThumb($foto){

        list($largura,$altura) = getimagesize($foto);

        $novaLargura = 300;
        $novaAltura = 300;

        $miniatura = imagecreatetruecolor($novaLargura, $novaAltura);
        $imagem = imagecreatefromjpeg($foto);
        imagecopyresampled($miniatura, $imagem, 0, 0, 0, 0, $novaLargura, 
        $novaAltura, $largura, $altura);
        imagejpeg($miniatura,$foto,50); // Já coloquei 100 e não funcionou
        return $foto;
}

I was able to decrease the image, but the quality is very poor. Check out one of the photos:

The photos are jpg and png. How could I improve the quality of the images?