Hi,
Now, without JCH, you can see that the mentioned CSS is on line 323 of templates/r_rnshop/scss/hydrogen/_hikashopstyles.scss, means it's coming from your Joomla template.
So, in your template's custom CSS file you should be able to add to it, or override other existing CSS. Adding the following to that custom CSS file will definitely not hurt:
body .hikashop_product_page .hikashop_small_image_div > a img {
width: auto;
}
After (!) you've finished all development, you should be able to activate JCH again -- hopefully without affecting appearance and functionality, but that also depends on your JCH settings.