@media only screen and (max-width: 650px) {
  body{
  background-image: url('back2.jpg') !important;
  background-repeat: no-repeat;
  background-attachment: fixed;
  background-size: 100% 100%;
  }

}
@media only screen and (min-width: 651px) {
body {
  background-image: url('back.jpg');
  background-repeat: no-repeat;
  background-attachment: fixed;
  background-size: 100% 100%;
 }
}