Switch default `img` attribute from `srcset` to `src`

Respect the HTML spec:

> The src attribute must be present, and must contain a valid non-empty URL potentially surrounded by spaces […]
> If the srcset attribute is present, the sizes attribute may also be present.

(Source: [WhatWG](https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element))

Improve degradability: even without a polyfill, or with failing JS, older browsers will still display the default image.