Hi,
You need to edit the different view files where images are output and add the width and height attributes to the img tag there.
For example, in the file show_block_img, you can edit the line:
$html = '<img id="hikashop_main_image'.$variant_name.' "itemprop="image" style="margin-top:10px;margin-bottom:10px;display:inline-block;vertical-align:middle" title="'.$this->escape(@$image->file_description).'" alt="'.$this->escape(@$image->file_name).'" src="'.$img->url.'"/>';
and use $img->width and $img->height