html {
background: url(https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSvctt0I--skoKQVfuVt-i4Et6vQ5ve7lXPkLy9sTElCWLKh1Ps) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
Know more about background-size
here.