Hi @nicmare,
This is something you can hack using the acf/fields/relationship/query filter. This filter will allow you to modify the $args array used to query the posts. For instance, should you want to query only posts which have featured image set, then you could use the following code:
add_filter('acf/fields/relationship/query', 'my_relationship_query', 10, 3);
function my_relationship_query( $args, $field, $post ){
$args['meta_query'] = array(
'key' => '_thumbnail_id',
'compare' => 'EXISTS'
);
return $args;
}
For more info on this, have a look at
