.background {
    background-image: url("../bk.gif") ;
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center center;
    height: 100%;
    background-attachment: fixed;
}

@media only screen and (max-width: 575px){
    /*xD*/
    .background{

          background-image: url("../bk.gif") ;
        background-repeat: no-repeat;
        background-size: cover;
        background-position: center center;
        height: 100%;
        background-attachment: fixed;
    }
    
}