Hi,
I'm Guillermo from Moonsoft support, we think this issue should be handled here as we're the ones asking for help about hikashop .
The workflow for this specific case is: the calculator adds product information to the table #__hikashop_product. Then redirects (using as you say, add to cart url) to the checkout. When we see images stored for a product in hikashop, we find the information inside the table #__hikashop_file instead, this is the 'other' table we mentioned. Or we're wrong and there is any other way to set the product image using only the product table?
As the calculator is not ready to manage the #__hikashop_file table by default, we suggested our user to confirm with you if any valid html tag, like an img with valid absolute url for ex, could be stored on a product custom field, so we can update it easily at #__hikashop_product table. as we were not sure if every part of the system, checkout, invoices, etc, will show properly this kind of information.
Thanks
Regards