Preloader issue

what cause showing preloader loading.gif on product page just on page load, before clicking large image?
What is wrong with jquery.fancybox-1.3.4.js and css?

It may be having compatibility issues. You’re using fancybox 1.3.4 there, and it’s progressed onwards now to version 2.

no, Prestashop uses fancybox 1.3.4 by default, so this not a reason.

The fancybox 1.3.4 library file that your site uses is not the same as the one that comes from official sources.
Your library file has been edited, with parts commented out, and other parts changed.

I would revert the library code back to the official code, from which troubleshooting can then successfully take place.

yes, original fancybox JS and CSS has been edited in order to replace the default preloader image with custom one. I’m trying to find out why it don’t work correctly.

With something as complex as the fancybox library, I do not troubleshoot that when its library file has been edited and modified.