body {
    margin: 0px;
} 
.main {
    width: 100%;
    height: 100%;
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    background-image: url("images/miciamici.jpg");
    background-size: cover;
    background-position: center;
}