rel= attribute?

It seems nobody knows how to do this, so I have reverted to Image Widget 3.3.4 which still works with WP 3.5.1.

With some guesswork I have figured this out, so if anyone else comes looking, this seems to work for v. 4.0.6

In image-widget.php, around line 254 replace

$output = '<a';

with

$output = '<a rel="shadowbox"';

i made a contribution to the plugin beacuase i had the same problem

https://github.com/caminantestelar/image-widget

i hope it helps

thank you caminantestelar, this is excellent
lets the hope ‘Modern Tribe’ incorporate it in the next release