'post',
'posts_per_page' => 20,
'tax_query' => array(
'taxonomy' => 'post_format',
'field' => 'slug',
'terms' => 'post-format-image',
));
// the query
$the_query = new WP_Query( $args );
if ( $the_query->have_posts() ) :
while ( $the_query->have_posts() ) : $the_query->the_post(); ?>