NextGEN echo gallery pictures

<ul>
<?php 
// nggdb::get_gallery( <gallery ID> )
// Get an array of nggImage objects.
$slides = nggdb::get_gallery( 1 ); 
foreach( $slides as $slide ) {
  // To see all the methods and attributes available, nextgen-gallery/lib/image.php
  echo '<li><img src="'.($slide->cached_singlepic_file( 872, 331 )).'" alt="'.esc_attr($slide->alttext).'" /></li>';
}
?>
</li>