.back {
    position: absolute;
    width: 100%;
    height: 100%;
    margin: 0px;
    padding: 0px;
    z-index: -1;
    background: url(../img/back.jpg) no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}