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 = “这里添加默认图片的路径，文章中没有图片时显示”;
    }
    return $first_img;
}

<img src=“<?php echo catch_that_image() ?>” alt=“” />