<style>
  html, body {
    width: 100%;
    height: 100%;
    margin: 0;
  }

  body {                    例：ファイル名 ↓
    background-image: url("Wallpaper0001.JPG");
    background-size: cover;
    background-position: center center;
    background-repeat: no-repeat;
    background-attachment: fixed;
  }
</style>