Error is most likely your lightbox mod
according to your header
/* Zen Lightbox v1.5 2008-12-18 */ (chrome browser had only just been released as beta at that point)
And the entire js is emmbedded in the header rather than as a linked file
update to the latest version.
Javascript engines have been evolving a lot, i know for image handler we have had to make changes for chrome, safari, opera and IE9, so a 3 year old light box script probably needs updating.
Have a look here
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=273
they also have a support thread
http://www.zen-cart.com/forum/showthread.php?t=45314&page=70
Its always best to post in the support thread as the people who maintain the add-on tend to look there, and modern version of the add-on works perfectly with imagehandler 3