html, body {
width: 100%;
height: 100%;
margin: 0px; /* retirando a margem padrão */
padding: 0px; /* retirando o padding padrão */
background-image:url(bg.jpg)
}

#central {
width: 700px;
height: 480px;
top: 50%;
left: 50%;
margin-top: -240px;
margin-left: -350px;
position: absolute;
}