body.login_admin {
  background: url('https://yourdoamin.com/media/login_custom.png') no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}

body.customers_login {
  background: url('https://yourdoamin.com/media/login_custom.png') no-repeat center center fixed;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}