tax_query "OR" relation -> no posts are extracted

Hello everybody,

I’m building a theme for my personal website and made a custom taxonomy for photos.
I’m trying to extract posts from that taxonomy, with an “OR” relation. Later on I use get_posts($args)

$args = array(
        'post_type' => 'attachment',
        'post_mime_type' =>'image',
        'posts_per_page' => -1,
        'tax_query' => array(
            'relation' => $a['rel'], // Here's the problem
            $tq
        ),
        'orderby' => $orderby,
        'order' => $order
    );

This is the arguments array. $tq is an array of arrays containing each term I’m trying to relate with an “or”.
It works until I try to use 'relation' => 'OR'

Any help will be very appreciated.

Thanks in advance