Hi,
That's indeed the case. HikaShop needs to generate thumbnails of the images in order to display the images properly on all the views. So it must have the image on hand in order to do that.
The alternative would be to create a custom product field of the type "text" where you would store the link to the image of the product and then customize the different views (product/listing_img_title, product/show_block_img, checkout/cart) and emails of HikaShop in order to display the content of the field inside an img tag instead of the code there to generate the thumbnails and display them.