Impostare layout responsive mobile device

[inkemodo]

parte del css... una cosa che non riesco a capire è perche sugli S4 lo sfondo non viene visualizzato...
non ho impostato bene tutti i parametri del css perchè vorrei risolvere prima il problema dell's4 poi devo cambiare i css in base alle varie risoluzioni. inoltre volevo capire quali dimensioni in px devo usare per creare le immagini di sfondo...quali sono le misure standard da utilizzare?

body {
  /* Location of the image */
  background-image: url('images/sfondo.jpg');
  
  /* Image is centered vertically and horizontally at all times */
  background-position: center center;
  
  /* Image doesn't repeat */
  background-repeat: no-repeat;
  
  /* Makes the image fixed in the viewport so that it doesn't move when 
     the content height is greater than the image height */
  background-attachment: fixed;
  
  /* This is what makes the background image rescale based on its container's size */
  background-size: cover;
}

/* Smartphones (portrait and landscape) ----------- */
@media only screen and (min-device-width : 320px) and (max-device-width : 480px) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-112px; 
margin-top: -200px; 
padding:0; 
}


}


/* Smartphones (landscape) ----------- */
@media only screen and (min-width : 321px) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}


/* Smartphones (portrait) ----------- */
@media only screen and (max-width : 320px) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}


/* iPads (portrait and landscape) ----------- */
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}


/* iPads (landscape) ----------- */
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px) and (orientation : landscape) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}


/* iPads (portrait) ----------- */
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px) and (orientation : portrait) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}
/**********
iPad 3
**********/
@media only screen and (min-device-width : 768px) and (max-device-width : 1024px) and (orientation : landscape) and (-webkit-min-device-pixel-ratio : 2) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}


@media only screen and (min-device-width : 768px) and (max-device-width : 1024px) and (orientation : portrait) and (-webkit-min-device-pixel-ratio : 2) {
/* Styles */
}
/* Desktops and laptops ----------- */
@media only screen  and (min-width : 1224px) {
body {
    /* The file size of this background image is 93% smaller
     * to improve page load speed on mobile internet connections */
   background-image: url('images/sfondo-mobile.png');
    
    
}


.wrapper {
    width: 100%;
    height:400px;
    border: 0px solid black;
    position:absolute;
    background-image: url('images/logo.png');
    background-repeat: no-repeat;
    background-size: contain;
    border: 0px solid;
    position:absolute;
top:50%; 
left:50%; 
margin-left:-212px; 
margin-top: -200px; 
padding:0; 
}


}