It's caused by some CSS within a media query for any screen widths below 1200px. It's not originating from HikaShop, so I wonder where it's coming from as the RCATheme template demo does not show this behaviour. You may want to make sure you have the latest template version, and/or to contact the template creators to verify it's not them responsible for the CSS.
Anyway, for a quick fix you'll need to edit the file /templates/rn-explorer/css-compiled/master.css, where somewhere around 2/3 into it there's this code:
.hikashop_product_image{height:100% !important;}
You should replace it with
.hikashop_product_image{height:auto;}
Note:
Depending on what really causes it, it's likely that the problem will re-occur if you make changes in the template's backend or update it. So, again, you may also want to contact the template creators to see what the cause is, and for it to be avoided in the future.