nice one but this is the correct query:
// only show results with image in relationship field
add_filter('acf/fields/relationship/query', 'my_relationship_query', 10, 3);
function my_relationship_query( $args, $field, $post ){
$args['meta_query'] = array(
array(
// just check if this key exists:
'key' => '_thumbnail_id'
)
);
return $args;
}
thank you for leading me into the right direction!