Forward and backward

Hi all.
Main problem I’m facing is that trying to create a really simple img gallery (as per the page link above mentioned), the ‘displayed linked file’ is just the image itself.
No way to move forward or backward among all the others. Also, the only chance I have to exit from the photo I clicked on is the back browser button….
Where’s my error ?
Thanks in advance.

The page I need help with: [log in to see the link]