Result Size: 300 x 150
x
 
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
/* Для устройства шириной менее 400px: */
body {
  background-repeat: no-repeat;
  background-image: url('../images/img_smallflower.jpg');
}
/* Для устройства шириной 400px и больше: */
@media only screen and (min-device-width: 400px) {
  body {
     background-image: url('../images/img_flowers.jpg');
  }
}
</style>
</head>
<body>
</body>
</html>