body, html {
  margin: 0;
  padding: 0;
  height: 100%;
}

/* 默认 PC 端背景图 */
.responsive-image {
  background-image: url("images/image1.jpg");
  background-size: cover;
  background-position: center;
  width: 100%;
  height: 100vh;
}

/* 手机端（宽度 ≤ 768px）显示另一张图 */
@media (max-width: 768px) {
  .responsive-image {
    background-image: url("images/image2.jpg");
  }
}