Edit in JSFiddle

<div class="container">
    <span class="img center"></span>
</div>

.container {
    background-color: #eee;
    height: 200px;
    position: relative;
}
.img {
    display: inline-block;
    width: 64px;
    height: 64px;
    background-color: green;
}
.center {
    position: absolute;
    top: 0; left: 0; bottom: 0; right: 0;
    margin: auto;
    overflow: auto;
}