body, html {
  height: 100%;
}
body {
  margin: 0;
  padding: 0;
  background: #FFFFFF;
  background-image: url(./images/mauerwerk_10_400px.jpg);
}
.mainDiv {
  position: relative;
  width: 99%;
  height: 99%;
}
.subDiv {
  position: absolute;
  width: 478px;
  height: 358px;
  top: 50%;
  left: 50%;
  margin-top: -179px;
  margin-left: -239px;
  background-image: url(./images/baustelle.gif);
}
.subDiv img {
  border: 0;
}