body {
    background: url(images/background_soon.jpg)no-repeat center center fixed;
                -webkit-background-size: cover;
                -moz-background-size: cover;
                -o-background-size: cover;
                background-size: cover; 
                background-size: 100% 100%;
  }

@media only screen and (max-width: 400px){    
body {
    background: url(images/background_soon_alt.jpg)no-repeat center center fixed;
                -webkit-background-size: cover;
                -moz-background-size: cover;
                -o-background-size: cover;
                background-size: cover; 
                background-size: 100% 100%;
 }
}