I would prefer to solve this without installing more plugins. Anyway, the change you made in product-image.php has lead to another problem: Now, the add-to-cart-popup doesnt show the product image anymore.
Here is the original ROYAL code:
<?php echo apply_filters( 'woocommerce_single_product_image_html', sprintf( '<a href="%s" itemprop="image" class="woocommerce-main-image product-image" data-o_href="%s" data-o_srcset="%s" data-o_src="%s" title="%s">%s</a>', $image_link, $image_link, $image_srcset, $image_src[0], $image_title, $image ), $post->ID ); ?>
And here is what you changed in order to remove the error message:
<?php echo apply_filters( 'woocommerce_single_product_image_html', sprintf( '<a href="%s" itemprop="image" class="woocommerce-additional-image product-image" data-o_href="%s" data-o_srcset="%s" data-o_src="%s" title="%s">%s</a>', $image_link, $image_link, $image_srcset, $image_src[0], $image_title, $image ), $post->ID ); ?>
-> class=”woocommerce-additional-image product-image” instead of class=”woocommerce-main-image product-image”. But as said, the product image is not shown in the emodal anymore.
Any idea how to fix this?