body {
 background-image: url(mandalamed.jpg), url(issarenskew.jpg);
 background-color: red;
 height: 630px;
 background-position: center;
 background-repeat: no-repeat;
}

.box {
    width:50px;
    height:50px;
    background-color:#000000;
    position:fixed;
    margin-left:-25px; /* half of width */
    margin-top:-25px;  /* half of height */
    top:50%;
    left:50%;
}


* {
  box-sizing: border-box;
}