文件路径 ripro/inc/theme-functions.php 函数名称 _get_post_thumbnail_url ， 直接覆盖原来的函数即可

function _get_post_thumbnail_url($post = null){
    if ($post === null) {
        global $post;
    }
    // cao_is_gif()
    if (has_post_thumbnail($post)) {
        //如果有特色缩略图，则输出缩略图地址
        $image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'full');
        $post_thumbnail_src = $image[0];
    } else {
        //查看post表中当前文章上传的图片，找第一个为缩略图
        global $wpdb;
        $post_thumbnail_src = '';
        $resulsst = _the_theme_thumb();
        $resulsst = $wpdb->get_results("SELECT guid FROM $wpdb->posts WHERE post_parent ='$post->ID'");
        if (!empty($resulsst)) {
            $post_thumbnail_src = $resulsst[0]->guid;
        } else {
            $post_thumbnail_src = _the_theme_thumb();
        }
    }
    return $post_thumbnail_src;
}

可能标点符号在编辑器有转化，下面是源文件；theme-functions

=============================8.0==========

 else{
    $post_thumbnail_src = '';
    //查看post表中当前文章上传的图片，找第一个为缩略图
    global $wpdb;
    $resulsst = _the_theme_thumb();
    $resulsst = $wpdb->get_results("SELECT guid FROM $wpdb->posts WHERE post_parent ='$post->ID'");
    if (!empty($resulsst)) {
        $post_thumbnail_src = $resulsst[0]->guid;
    } else {
        $post_thumbnail_src = _the_theme_thumb();
    }
}
return $post_thumbnail_src;