rel= attribute?

Until this latest version I have been able to add rel="shadowbox" to

echo '<a>widget_options['classname'].'-image-link" href="'.esc_url($link).'" target="'.esc_attr($linktarget).'">';

in widget.php but now I can’t see how to achieve the same effect. I would appreciate some guidance please.

http://wordpress.org/extend/plugins/image-widget/