#banner-back{
    left:0px; 
    margin: 0 auto; 
    min-height:1080px; 
    min-width:990px; 
    padding:0; 
    position:absolute; //position:absolute, position:fixed (para mantener el fondo fijo
    top:0px; 
    width:100%;
} 
.banner-crop{
    height:100%; 
    left:0; 
    overflow:hidden; 
    position:absolute; 
    top:0; 
    width:100%; 
}
.banner-center{
    left:50%;
    position:absolute;
    top:0;
}
.banner-offset{
    left:-50%;
    position:relative;
    top:0;
}
.banner-img{
    display:block;
    margin:0 auto;
    text-align:center;
}