$("#myModal .post_content img").each(function(){

    //get the imgs url
    var imgSrc = $(this).attr("src");

     //put the image inside a div
    $(this).wrap('<div class="lightgallery"></div>');

    //adds the button to launch the lightbox (include thumbnail)
    $(this).prepend('<a href="'+imgSrc+'"><img data-sub-html="'.get_post(get_post_thumbnail_id())->post_excerpt.'" class="slider-image-fullscreen" data-src="'.get_the_post_thumbnail_url($postid,'full').'" src="'.get_template_directory_uri().'/images/Magnifier.png"></a>');

});