Add

'class' => 'elgg-lightbox',

BTW, elgg_load_js('lightbox'); and elgg_load_css('lightbox'); are deprecated since 2.2

RvRThat did not work, but replacing categories-popup with elgg-lightbox worked.

Thanks for your help RvR.

hi @Rudi sorry to but in, but where did you download this from ? it goes to a blank page 

NB: Go to http://www.elggstore.webintelligence.ie/ and download the Web Intelligence Framework. Will not work without our framework.

it says this will not work without the download :/ 

Thank you very much Rohit Gupta, That worked.

Except the lightbox popup does not display the image, will look in to that.

viewimage.php had the same engine/start.php in it, so replaced:

require_once(dirname(dirname(dirname(__FILE__))) . "/engine/start.php");

with this:

$autoload_root = dirname(__DIR__);
 if (!is_file("$autoload_root/vendor/autoload.php")) {
 $autoload_root = dirname($autoload_root);
 } 
require_once "$autoload_root/vendor/autoload.php";
 \Elgg\Application::start();

Thanks  Rohit Gupta, That also worked.

Now the lightbox popup does display the image.

Had also to tweak the size of the images to display right in the lightbox.

I am glad that it worked.