html, body{
  height: 100%;
  margin: 0;
  padding: 0;
    background-image: url("bkground.gif");
  text-align: center;
}



.imagem 
{
 width: 600px;
  height: 600px;
  position: absolute;
  top: 50%;
  left: 50%;
  margin-top: -300px; /* Metade da altura */
  margin-left: -300px; /* Metade da largura */
}