Best way to programatically add "rel" attributes to page and post images

Question

For single posts and pages, I would like to add certain rel attributes to all the images on the contained therein added with the "Add Image" functionality on the edit screen.

The attribute would be rel="lightbox[post id]"

What function would I filter to accomplish this most effectively?

Answer 1

image_send_to_editor

function insert_img_rel_attrib( $html, $id, $caption, $title, $align, $url, $size, $alt )
{
  /* Run your magic on the image $html to insert rel attrib */
  return $html;
}
add_filter( 'image_send_to_editor', 'insert_img_rel_attrib' );

Answer 2

The rel="whatever" attribute is only for search engines. If you consider to make your content "better" for spiders, crawlers and such, take a look at this list first.