The reason your full size images aren't displaying is because your path is wrong.
<a class="fancybox" data-fancybox-group="gallery" href="GALLERY/slides/00024.jpg"
it should probably be
<a class="fancybox" data-fancybox-group="gallery" href="Gallery/slides/00024.jpg"
Fix the PHP code on or about line #138 in code view.
<a class="fancybox" data-fancybox-group="gallery" href="GALLERY/slides/'.$file.'" title="'.$title.'">'.$title.'</a>
I think you can work out the rest on your own. So I'm going to assume this thread is answered.
Good luck!
Nancy O.