Edit in JSFiddle

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

.container {
    background-color: #eee;
    height: 200px;
}
.img {
    display: inline-block;
    width: 64px;
    height: 64px;
    background-color: green;
}
.center {
    display: flex;
    justify-content: center;
    align-items: center;
}