Ok, I'm one step further now. I discovered that uploaded files were
saved to my uploads directory without read permissions.
So I added:
chmod("../uploads/".sanitizeName($_FILES['photo']['name']), 0644);
to admin/bm.php and admin/index.php
Now the files have read permissions an I can access them directly by
going to the uploads directory.
My hope was this would solve the lightbox problem, but it didn't.
Lightbox still doesn't load the images, it just sits there loading.