Hi,
An "alt" is something added to the img tag on an HTML page. The image file itself doesn't have an "alt".
And so on product pages, HikaShop does add the alt attribute to the img tag of the main original image file at upload/xxx.jpg.
It's done by the code
$html = '<img id="hikashop_main_image'.$variant_name.'" '.$attributes.' title="'.$this->escape(@$image->file_description).'" alt="'.$this->escape(@$image->file_name).'" src="'.$img->url.'"/>';
in the file show_block_img that you can edit via the menu Display>Views.
So I don't see what we can do.
To me, it just looks like your site analyzer doesn't know what it's talking about, or you've modified the main image display of the product page to not include the alt attribute.