body { 
         background: url(../img/fondo_dif.jpg) no-repeat center center fixed; 
         -webkit-background-size: cover;
         -moz-background-size: cover;
         -o-background-size: cover;
         background-size: cover;
     }


#form_login{
background: url(../img/cuadrado.png) no-repeat center center fixed; 
         -webkit-background-size: cover;
         -moz-background-size: cover;
         -o-background-size: cover;
         background-size: cover;
         color: #FFFFFF;
}