首先在主题页眉文件header.php的head标签中添加css引用文件

<!-- 图片放大 -->
<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/fancyapps/fancybox@3.5.7/dist/jquery.fancybox.min.css" />

然后在主题页脚文件footer.php中添加js引用文件

<!-- 图片放大 -->
<script src="https://cdn.jsdelivr.net/gh/fancyapps/fancybox@3.5.7/dist/jquery.fancybox.min.js"></script>

最后在模板函数文件functions.php的最后面添加如下代码

/**图片灯箱自动给图片加链接**/add_filter('the_content', 'fancybox');
function fancybox($content){ 
    $pattern = array("/<img(.*?)src=('|\")([^>]*).(bmp|gif|jpeg|jpg|png|swf)('|\")(.*?)>/i","/<a(.*?)href=('|\")([^>]*).(bmp|gif|jpeg|jpg|png|swf)('|\")(.*?)>(.*?)<\/a>/i");
    $replacement = array('<a$1href=$2$3.$4$5 data-fancybox="gallery"><img$1src=$2$3.$4$5$6></a>','<a$1href=$2$3.$4$5 data-fancybox="images"$6>$7</a>');
    $content = preg_replace($pattern, $replacement, $content);
    return $content;
}