Edit in JSFiddle

<div class="wrapper">
    <img class="classname" src="http://placehold.it/150x150" alt="" />
</div>

.wrapper {
    position: relative;
    width: 100%;
    height:300px;
    border: 1px solid black;
    
}

img.classname{
   position:absolute;
   top:50%;
   left:50%;
   -webkit-transform: translate(-50%,-50%);
   -moz-transform: translate(-50%,-50%);
   -ms-transform: translate(-50%,-50%);
   -o-transform: translate(-50%, -50%);
   transform: translate(-50%,-50%);
}