Overrides NGG shortcode. Example for query.

functions.php

function gurustu_slideshow_override( $atts, $content = null ) {
	global $wpdb;
	$atts = shortcode_atts( array(
		'id' => '0',
	), $atts );
	if( $atts['id'] ){
		$gallery = 'SELECT title, path FROM wp_ngg_gallery WHERE gid = '. $atts['id'] .';';
		$pictures = 'SELECT description, filename FROM wp_ngg_pictures WHERE galleryid = '. $atts['id'].';';

		$gallery = $wpdb->get_results($gallery);
		$pictures = $wpdb->get_results($pictures);

		ob_start();

		// print_r( $gallery );
		// print_r( $pictures );
	?>
	<div class="clear"></div>
	<div class="inline-slideshow-wrap">
		
		<ul class="rs-slideshow">
			<?php foreach( $pictures as $pic ) : ?>
			<?php 

				$file = get_site_url() . '/' . $gallery[0]->path . '/' . $pic->filename;

			 ?>
			<li><a href="<?php echo $file; ?>" rel="lightbox[<?php echo $gallery[0]->gid; ?>]"><img src="<?php echo $file; ?>" alt="<?php echo $pic->description; ?>"></a></li>
			<?php endforeach; ?>
		</ul>
		<div class="inline-slideshow-nav">
		</div>
	</div>
	<?php
	}
	return ob_get_clean();
}

add_shortcode( 'slideshow', 'gurustu_slideshow_override' );