Hi,
Well, it is a little bit more complicated than modifying one line in the listing_table view.
The popup is generated by
echo $this->popup->image($html, $img->origin_url, null, $attr);
The first parameter is the content which is in the "<a ..>" link (so the thumbnail image in the product page).
The second parameter is the link to the original image ($link in the listing_table view).
The third parameter is not used for our case.
The last parameter is the attributes, like the class or the onclick.
In the product page, we set the onclick event which use the javascript function defined later in the view. The function will create the gallery effect (removing duplicate images) but if you don't want to have a gallery, you can set this parameter empty ; it should work good.
As you will understand, it requires some php/development knowledge.
Regards,