i marked the topic as solved because i found a workaround.
right now i am using no more lightbox anymore.
the easy fancybox plugin lets you create galleries in a div and hides the images from the main page.
whenever fancybox is opened the images will show up in addition to the portfolio image. pretty cool.
the theme must allow to do shortcodes in the excerpt for the portfolio - that's the trick of how i got it done this. time.
you can achieve this by adding:
add_filter( 'the_excerpt', 'shortcode_unautop');
add_filter( 'the_excerpt', 'do_shortcode');
to the functions.php of the theme.
all my best