How to theme the user picture to appear in lightbox I want to enable viewers to click on an image and have it displayed as a lightbox.

in my user-profile.tpl.php I have access to '$user_profile['picture']'. After I dsm($user_profile) I see an array of $user_picture with #markup and #weight elements.

I'm not quite sure if this is the right approach and how to go about altering the markup before it's displayed so i can add 'rel="lightbox"'
An alternative approach to wrapping the user picture around the necessary  tag for lightbox, would be to append a custom CSS class to the user picture. Then navigate to /admin/config/user-interface/lightbox2/automatic (D7), down to custom class images and type in the name of the CSS class you chose before.