Pagination problem after WP_Query with tag filtering

Finally I’ve figured out the correct coding for this problem:

<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$wp_query = new WP_Query();
$wp_query->query('tag=concurs&posts_per_page=".get_option("posts_per_page').'&paged=' . $paged);
?>
<?php if ($wp_query->have_posts()) : ?>
    <?php while ( $wp_query->have_posts() ) : $wp_query->the_post(); ?>  
[code for title, author, category, meta-info etc] 
<?php endwhile; ?>
 <div class="next-prev"><?php posts_nav_link( ' ', '<img src="' . get_bloginfo('stylesheet_directory') . '/img/pagina-precedenta.png" align="left" />', '<img src="' . get_bloginfo('stylesheet_directory') . '/img/pagina-urmatoare.png" align="right" />'); ?></div>  
<?php endif; ?>