html, body {
   background: #00aeef;
   height: 100%;
   width: 100%;
   margin: 0;
   padding: 0;
}
#mainpage {
   background: url(visite.jpg) top left;
   background-repeat: no-repeat;
   position: absolute;
   top: 50%;
   left: 50%;
   margin-left: -400px;
   margin-top: -250px;
   width: 800px;
   height: 450px;
   border: 0;
   OVERFLOW: hidden;
}