<a href="<?php echo wp_get_attachment_image_url($b['imagem'],'full'); ?>" data-fancybox="img" title="<?php echo get_field('titulo_logo_topo','options'); ?>">
    <?php echo wp_get_attachment_image($b['imagem'], 'thumb_portfolio', false, array('class' => 'logo img-responsive hvr-grow','title' => get_field('titulo_logo_topo','options'),'alt' => get_field('titulo_logo_topo','options'))); ?>
</a>



$().fancybox({
    selector : '.fancybox',
    loop     : true
});