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); //正则匹配文章中所有图片
$first_img = $matches [1] [0];

if(empty($first_img)){ //定义默认图片
$first_img = "/images/default.jpg"; //默认图片地址需自己设置
}
return $first_img;
}