.foo {
  background-image: url('https://foo.com/img/bar.png');
}

.foo {
  background-image: url('http://foo.com/img/bar.png');
}

.foo {
  background-image: url('//foo.com/img/bar.png');
}

.foo {
  background-image: url('/img/bar.png');
}

.foo {
  background-image: url('img/bar.png');
}

.foo {
  background-image: url('bar.png');
}

.foo {
  background-image: url('data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7');
}