Pimcore\Model\Asset\Document\ImageThumbnail::createConfig PHP Method

    protected function createConfig($selector)
    {
        $config = Image\Thumbnail\Config::getByAutoDetect($selector);
        if ($config) {
            $format = strtolower($config->getFormat());
            if ($format == "source") {
                $config->setFormat("PNG");
            }
        }
        return $config;
    }