Hi,
Please check the plugin and see if you right have the line:
$result .= '<table class="acymailing_content"><tr><td valign="top" style="padding-right:5px"><a target="_blank" style="text-decoration:none;border:0" href="'.$link.'" >'.$image->display($product->file_path,false,$product->product_name, '' , '' ,$config->get('thumbnail_x',100),$config->get('thumbnail_y',100)).'</a></td><td>'.$description.'</td></tr></table>';
Can you tell more about you image sizes configuration in HikaShop ?
For what I see in the code and for I tested, it should work properly. Maybe there is something different in your configuration.
If we can reproduce your problem on our side, it will be easier for us to understand and propose you the right fix.
Regards,