this feature was removed in the interest of speed (loading remote images can slow down your site) and security (loading remote images can be dangerous)
that said, this is possible and should be doable by adding a text field to your join form template <input type="text" name="banner_url">, though you might need to inspect sources/join.php to get the processing part to work nice, any changes should be made via a plugin using the "join_process_form" hook location.