Around line 250. Added NextGen Gallery support to Related Posts Thumbnails WordPress plugin.

related-posts-thumbnails.php

<?php
if ( current_theme_supports( 'post-thumbnails' ) ) { // using built in Wordpress feature
    $post_thumbnail_id = get_post_thumbnail_id( $post->ID );
    $debug .= 'Post-thumbnails enabled in theme;';
    if ( !( empty( $post_thumbnail_id ) || $post_thumbnail_id === false ) ) { // post has thumbnail
        $debug .= 'Post has thumbnail '.$post_thumbnail_id.';';
        $debug .= 'Poststhname: '.$poststhname.';';
        // Added by MK for nextgen gallery support.
        $image = nggdb::find_image(substr($post_thumbnail_id, 4));
        $url = $image->cached_singlepic_file(150, 150, 'crop');
        $from_post_body = false;
    }