Modifying the HTML Output of Inserted Images

This filter checks if an anchor is in the html image send to the editor and if so adds rel and class to the anchor.

function add_highslide_attr( $html ) {
    preg_match_all('/(<a[^>]*>)(.*)/i', $html, $result);

    if( !isset($result) || !isset($result[1][0]) || ! $result[1][0] )
        return $html;

    $anchor = $result[1][0];

    if( strstr($anchor, 'rel=") )
        $anchor = str_replace("rel=""https://wordpress.stackexchange.com/questions/77460/,"rel="highslide ', $anchor);
    else
        $anchor = str_replace('href="https://wordpress.stackexchange.com/questions/77460/,"rel="highslide" href=", $anchor);

    if( strstr($anchor, "class=") )
        $anchor = str_replace("class=""https://wordpress.stackexchange.com/questions/77460/,"class="highslide ', $anchor);
    else
        $anchor = str_replace('href="https://wordpress.stackexchange.com/questions/77460/,"class="highslide" href=", $anchor);

    $html = $anchor . $result[2][0];

    return $html;
}
add_filter( "image_send_to_editor"https://wordpress.stackexchange.com/questions/77460/,"add_highslide_attr', 10 );