Hi,
First, please note that this is not as clear cut as this.
I invite you to read here:
stackoverflow.com/questions/32217863/seo...r-responsive-website
Specifying the height and width of the images can impact the responsiveness of the pages, which in turn will degrade SEO performances.
To add the width and height attributes to product images on listings, you can edit the view file product / listing_img_title.php via the menu Display>Views and change the line:
$html = '<img class="hikashop_product_listing_image" title="'.$this->escape((string)@$this->row->file_description).'" alt="'.$this->escape((string)@$this->row->file_name).'" src="'.$img->url.'"/>';
to:
$html = '<img class="hikashop_product_listing_image" title="'.$this->escape((string)@$this->row->file_description).'" alt="'.$this->escape((string)@$this->row->file_name).'" src="'.$img->url.'" width="'.$img->width.'" height="'.$img->height.'"/>';