You can use t-att-attribute_name to add an attribute try like:

<span itemprop="image" t-field="product.image" t-att-alt="product.name" t-field-options="{"widget": "image", "class": "product_detail_img shadow"}" ></span>

Edit:

Even if "alt" attribute is added you won't be able to see it on form, because on form it is never a case when there is no image, when you upload an image or give a wrong src for image(which doesn't exist) Odoo replaces that image to a default image which resides in "/web/static/src/img/placeholder.png" but it works well in Reports.

This works fine for me:

<span itemprop="image" t-field="bride.img_scaled_w400" t-options="{'widget': 'image', 'style':'maxwidth: 400px', 'alt-field': 'kco_alt'}" t-att-alt="bride.kco_alt" />

The alt attribute is returned in both the span and the img tags.  

If you want to add alt attribute for seo porposes. Add attribute alt-field in t-options.

 <span itemprop="image" t-attf-content="{{request.httprequest.url_root}}web/image/trending.template/{{trending.id}}/image" t-field="trending.image" t-options="{'widget': 'image','alt-field': 'name'}" t-att-alt="trending.name" />

thanks for the tip but as the alt attribute is included between span tags (and not img tags) this is not usefull for seo purposes