.bg {
  background: url("foo.png") no-repeat;
}
.bg-retina {
  background: url("foo.png") no-repeat;
  background-size: 300px 500px;
  width: 300px;
  height: 500px;
}
.bg-retina-repeat {
  background: url("foo.png") repeat center;
  background-size: 300px 500px;
}