body,html{
    background-image: url("https://www.yql.jp/imges/Background.png");
    background-size: cover;
    background-attachment: fixed;
    background-position: center center;
}

/* 小画面では固定を解除してパフォーマンスを改善 */
@media (max-width: 768px){
    body,html{
        background-attachment: scroll;
        background-size: cover;
    }
}