<amp-img
    media="(min-width: 700px)"
    src="/media/images/amp-bg.jpg"
    width="750"
    height="500" layout="responsive"></amp-img>
<amp-img
    media="(max-width: 699px)"
    src="/media/images/amp-bg-red.jpg"
    width="450"
    height="200" layout="responsive"></amp-img>