A simple shortcode to replace NextGens singlepic shortcode. You still need to have the nextgen tables within your database.

nextgen_singlepic_replacement.php

<?php

/**
 * This is a replacement for NextGens "singlepic" shortcode
 */

add_shortcode('singlepic', 'render_shortcode_nextgen_singlepic_replacement');

function render_shortcode_nextgen_singlepic_replacement($attributes = array()) {
    /**   @global wpdb $wpdb */
    global $wpdb;
    $attributes = shortcode_atts(array(
        'id' => -1,
        'w'  => -1,
        'h'  => -1,
    ), $attributes);

    if ($attributes['id'] == -1) {
        return __('Image not defined', 'textdomain');
    }


    $image = $wpdb->get_row('SELECT galleryid, filename FROM ' . $wpdb->prefix . 'ngg_pictures WHERE pid=' . intval($attributes['id']));
    if (!$image || $image == null) {
        return __('Image not found', 'textdomain');
    }

    $gallery = $wpdb->get_row('SELECT path FROM ' . $wpdb->prefix . 'ngg_gallery WHERE gid=' . intval($image->galleryid));

    if (!$gallery || $gallery == null) {
        return __('Gallery not found', 'textdomain');
    }

    $style = '';
    if (intval($attributes['h']) > 0) {
        $style .= 'height: ' . intval($attributes['h']) . ' ;';
    }
    if (intval($attributes['w']) > 0) {
        $style .= 'width: ' . intval($attributes['w']) . ' ;';
    }
    return '<img id="nextgen-origin-' . intval($attributes['id']) . '"style="' . $style . '" src="' . get_site_url() . '/' . $gallery->path . '/' . $image->filename . '" />';
}