<div class="image_box">
  <img src="https://placehold.jp/3d4070/ffffff/1920x640.png?text=image" srcset="https://placehold.jp/3d4070/ffffff/1920x640.png?text=image 1x,https://placehold.jp/3d4070/ffffff/1920x640.png?text=image 2x" alt="" />
</div>

.image_box img {
  width: 100%;
}
@media screen and (max-width: 680px) {
  .image_box img[src$="1920x640.png?text=image"] {
    content: url(https://placehold.jp/70403d/ffffff/680x840.png?text=SP_image);
  }
}