public function getThumbnail($size = self::IMG_SIZE_THUMBNAIL)
    {
        /*
         * Define a func for to get the attachment-src from the post_id
         */
        $getSrc = function ($_id) use ($size) {
            $imageObject = wp_get_attachment_image_src(get_post_thumbnail_id($_id), $size);
            if (empty($imageObject)) {
                return false;
            }
            return $imageObject[0];
        };

        if (($imageObject = $getSrc($this->ID))) {
            return $imageObject;
        }
        // if they aren't, we get the first img from the post, and let it as thumbnail
        preg_match('/< *img[^>]*src *= *["\']?([^"\']*)/i', $this->post_content, $matches);
        $src = isset($matches[1]) ? $matches[1] : '';
        $attachmentId = Utils::getAttachmentIdFromUrl($src);
        // try to set the first img as thumbnail
        set_post_thumbnail($this->ID, $attachmentId);
        /*
         * In case we found it return the img src from wp_get_attachment_image_src()
         * Othercase return the raw source
         */
        if (($imageObject = $getSrc($this->ID))) {
            return $imageObject;
        }

        return $src;
    }