我用这个找到了解决方案How to change the markup Wordpress inserts for post images

我的最终代码是：

add_filter( \'image_send_to_editor\', \'add_custom_data_attribute_send_to_editor\', 10, 8 );
function add_custom_data_attribute_send_to_editor( $html, $id, $caption, $title, $align, $url, $size, $alt ){  
    if( $id > 0 ){
        $post = get_post( $id );
              $html = str_replace( "<img src", "<h2>$alt</h2><img src", $html );
    }
    return $html;
}