Hi, in the end I found the solution, in the product.tpl file (coming from a prestashop template) I have this:
{if $have_image}
<span id="view_full_size">
{if $jqZoomEnabled && $have_image && !$content_only}
<a class="jqzoom" title="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}" rel="gal1" href="{$link->getImageLink($product->link_rewrite, $cover.id_image, 'thickbox_default')|escape:'html':'UTF-8'}">
<img itemprop="image" src="{$link->getImageLink($product->link_rewrite, $cover.id_image, 'large_default')|escape:'html':'UTF-8'}" title="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}" alt="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}"/>
</a>
{else}
<img id="bigpic" itemprop="image" src="{$link->getImageLink($product->link_rewrite, $cover.id_image, 'large_default')|escape:'html':'UTF-8'}" title="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}" alt="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}" width="{$largeSize.width}" height="{$largeSize.height}"/>
{if !$content_only}
<span class="span_link no-print">{l s='View larger'}</span>
{/if}
{/if}
</span>
{else}
<span id="view_full_size">
<img itemprop="image" src="{$img_prod_dir}{$lang_iso}-default-large_default.jpg" id="bigpic" alt="" title="{$product->name|escape:'html':'UTF-8'}" width="{$largeSize.width}" height="{$largeSize.height}"/>
{if !$content_only}
<span class="span_link">
{l s='View larger'}
</span>
{/if}
</span>
{/if}
Instead of this:
{if $have_image}
<a class="fancybox"
data-fancybox-group="product"
id="view_full_size"
href="1"
onclick="return false;"
>
<noscript>
<img class="img-responsive center-block"
itemprop="image"
src="{$link->getImageLink($product->link_rewrite, $cover.id_image, 'large_default', null, ImageManager::retinaSupport())|escape:'html':'UTF-8'}"
title="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}"
alt="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}"
width="{$largeDefaultWidth|intval}"
height="{$largeDefaultHeight|intval}"
>
</noscript>
<picture id="bigpic">
<!--[if IE 9]><video style="display: none;"><![endif]-->
{if !empty($webp)}
<source class="img-responsive center-block"
itemprop="image"
srcset="{$link->getImageLink($product->link_rewrite, $cover.id_image, 'large_default', 'webp', ImageManager::retinaSupport())|escape:'html':'UTF-8'}"
title="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}"
type="image/webp"
>
{/if}
<!--[if IE 9]></video><![endif]-->
<img class="img-responsive center-block"
itemprop="image"
src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYAAAAAYAAjCB0C8AAAAASUVORK5CYII="
srcset="{$link->getImageLink($product->link_rewrite, $cover.id_image, 'large_default', null, ImageManager::retinaSupport())|escape:'html':'UTF-8'}"
title="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}"
alt="{if !empty($cover.legend)}{$cover.legend|escape:'html':'UTF-8'}{else}{$product->name|escape:'html':'UTF-8'}{/if}"
width="{$largeDefaultWidth|intval}"
height="{$largeDefaultHeight|intval}"
>
</picture>
{if !$jqZoomEnabled && !$content_only}
<span class="span_link" title="{l s='Zoom in'}">
<i class="icon icon-search-plus"></i>
</span>
{/if}
</a>
{else}
<span id="view_full_size">
<noscript>
<img class="img-responsive center-block"
itemprop="image"
src="{$img_prod_dir|escape:'html':'UTF-8'}{$lang_iso|escape:'html':'UTF-8'}-default-large_default.jpg"
title="{$product->name|escape:'html':'UTF-8'}"
width="{$largeDefaultWidth|intval}"
height="{$largeDefaultHeight|intval}"
>
</noscript>
<picture id="bigpic">
<!--[if IE 9]><video style="display: none;"><![endif]-->
{if (!empty($webp))}
<source class="img-responsive center-block"
itemprop="image"
srcset="{$img_prod_dir|escape:'html':'UTF-8'}{$lang_iso|escape:'html':'UTF-8'}-default-large_default.webp"
title="{$product->name|escape:'html':'UTF-8'}"
type="image/webp"
>
{/if}
<!--[if IE 9]></video><![endif]-->
<img class="img-responsive center-block"
itemprop="image"
src="{$img_prod_dir|escape:'html':'UTF-8'}{$lang_iso|escape:'html':'UTF-8'}-default-large_default.jpg"
srcset="{$img_prod_dir|escape:'html':'UTF-8'}{$lang_iso|escape:'html':'UTF-8'}-default-large_default.jpg"
title="{$product->name|escape:'html':'UTF-8'}"
width="{$largeDefaultWidth|intval}"
height="{$largeDefaultHeight|intval}"
>
</picture>
</span>
{/if}