In the cofniguration you should set the product page image size to 390px if you want that size.
Then, in the default CSS there is this code:
div#hikashop_product_left_part img.hikashop_child_image{
margin : 2px;
height:25px;
}
For the height of the thumbnails on the product page.
That's all there is to it.
Also, I see that you have a small thumbnail of 200x169 where the image path is: media/com_hikashop/upload/thumbnail_325x385/saszka-biaamala-2-small_1608520945.png
So the thumbnail should be 325x385. You must have uploaded it manually right ?
Or your original image is 200x169 and in that case it is displayed "as is" (stretching is not an option). In that case, you should upload a bigger image.