vant/packages/zanui-css/src/image_preview.css

@component-namespace zan {
  @b image-preview {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    overflow: scroll;

    @e image {
      display: block;
      width: 100%;
      height: auto;
      transition: .2s;
      position: absolute;
      left: 0;

      @m center {
        top: 50%;
        transform: translate3d(0, -50%, 0);
      }

      @m top {
        top: 0;
        transform: none;
      }
    }
  }
}