Hi,
So, I looked further into it, and the issue comes from your template's customization of the results page of the Smart search. The view overrides in the template don't display the image.
If I switch to the default template of Joomla and clear the cache on your website, I do get the images for the HikaShop products :
i.imgur.com/Dc0WRhn.png
Since you said that it works for Joomla articles, I suppose it means that contrarily to what Joomla does by default, the template has custom code to load the images of the articles for the search results and that isn't compatible with other content.
I'm afraid you'll have to contact your template provider for help as I'm not sure what should be done in your template to properly support images display the same way Joomla does by default.