thumbnail image quality

[smallfish]

The thumbnail image quality is not good. Where can I increase the quality of the thumbnail?
Thank you!

[abantecart]

Thumbnail images are based on your source images. Load better quality source (main) images and thumbnails will be generated with better quality.

[smallfish]

OK, I find that the image class which is already used 100% as the quality.
Then, I really need to improve the quality of the source image file.
Thanks!

public function save($file, $quality = 100) {
      $extension = pathinfo($file,PATHINFO_EXTENSION);
        if ($extension == 'jpeg' || $extension == 'jpg') {
            imagejpeg($this->image, $file, $quality);