<?php

function catch_that_image() {

      global $post, $posts;

      $first_img = '';

      ob_start();

      ob_end_clean();

      $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i',

$post->post_content, $matches);

      $first_img = $matches [1] [0];

      if(empty($first_img)){ //Defines a default image

        $first_img = "/images/default.jpg";

      }

      return $first_img;
    }
?>
在要调用图片的地方加入代码：
<img decoding="async" src="<?php echo catch_that_image() ?>" />