Ok ... I test few things on your website and the following appears :
- I do not know which plugins does that but you have a caching feature activated on your website as, when I modify code, the appeareance is unchanged ...
- Either your installation of wordpress is buggy, or a plugin screws the WP functions.
For instance, I tried to display 2 results :
echo plugin_dir_url("/") ;
echo plugin_basename($this->path) ;
The previous code should display something like :
Instead the wp function on your website display
As you see it is very very different and that cause the image zoom plugin to bug ...
These function are not plugin's functions but are WP functions and their behaviour are well documented (you could have found a WP bug but it far beyond my expretise)
In any way, I recommend deactivating ALL plugins to see if that change something (and try the above code).
I have not try to deactivate any of the activated plugins as we have not discussed that before.