By default, WordPress allows authors/users to see all images uploaded to a WordPress site’s media library. This could be a problem if your site has users with individual images/galleries. This WordPress snippet will allow you to restrict WordPress media library access to the user’s own uploads.

Instructions
Add this code to your WordPress theme function.php file.

<?php // Limit access to media library (users can only see/select own media) //
   add_filter( 'ajax_query_attachments_args', 'wpsnippet_show_current_user_attachments' );
   function wpsnippet_show_current_user_attachments( $query ) {
      $user_id = get_current_user_id();
      if ( $user_id && !current_user_can('activate_plugins') && !current_user_can('edit_others_posts')) {
         $query['author'] = $user_id;
      }
      return $query;
   }
?>