About using jQuery to do this.

Everything is ok when I choose my thumbnails generated by making wp gallery.

What I see is that .thumbnail is a class for <a> and not for <img> .

But when I add function:

function add_image_class($class){
$class .= ' obrazek';
return $class;
}
add_filter('get_image_tag_class','add_image_class');

and class is assigned to posted image, lightbox won’t load it.

Any idea how to add class to <a> of that inserted image?

I don’t know if I’m clear about it, sorry for my english.