while googling about the same issue I came upon your question and a similar question from the WordPress forums. You can control the output of the generated code via the image_send_to_editor filter like this:
function filter_image_send_to_editor($html, $id, $caption, $title, $align, $url, $size, $alt) {
$html = sprintf('<a href="#" data-rel="lightbox-0"><img alt="%2$s" src="%1$s" /></a>', esc_attr(esc_url($url)), esc_attr($title));
return $html;
}
add_filter('image_send_to_editor', 'filter_image_send_to_editor', 10, 8);
Original link: https://wordpress.org/support/topic/change-generated-html-code-from-add-media-button