Best way to programatically add “rel” attributes to page and post images

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' );