Hi,
The data sent back by the add to cart system when you click on an add to wishlist button while not being logged in doesn't contain the product information, only the error message.
That's why you get the generic icon.
If you want the image of the product, you would have to modify the code:
$ret = array(
'ret' => 0,
'message' => JText::_('LOGIN_REQUIRED_FOR_WISHLISTS')
);
echo json_encode($ret);
exit;
in the file components/com_hikashop/controllers/product.php
However, that's more complex than changing the inside of the notification HTML since you need to load the product image data based on the REQUEST parameters, generate the thumbnail URL based on that, etc ...
The generation of the thumbnails and how to add it to the response can be seen in the getAjaxCartData function of the same file. Loading the product image data base be done with a MySQL query to the hikashop_file table.