Hi,
Well, it's not impossible to add the product image, however the data of the image is not loaded there.
So you would have to first run a MySQL query on the hikashop_file table to load the image of the product and then use the standard code you can find in the preload of the order notifications to generate the thumbnail URL out of the image data loaded from the database.
That will require a developer to help you with that if you're not a developer yourself.
You can potentially contact our partners for such customization:
www.hikashop.com/home/our-partners.html