问如何将CSS用于响应式站点，同时仍然在图像上提供ALT文本，以便我的站点可以访问？

EN

<picture>
 <source srcset="/images/header-mobile.png" media="only screen and (max-width: 480px)">
 <img src="/images/header-desktop.png" alt="Foo">
</picture>

img::before {
  content: "";
  background-image: url('http://lorempizza.com/200/200');
  width: 200px;
  height: 200px;
  display: block;
  position: absolute;
}

@media (max-width: 400px) {
  img::before {
    background-image: url('http://placehold.it/200X200')
  }
}