Edit in JSFiddle

<div class="box">
    <div class="button"> <a href="#">空要素</a>

    </div>
    <img src="http://wild-cards.net/wc/wp-content/uploads/2013/12/absolute.jpg">
</div>

.box {
    width: 300px;
    height: 100px;
    position: relative;
}
.button {
    position: absolute;
    bottom: 10px;
    right: 50px;
}
.button a {
    display: block;
    width: 50px;
    height: 20px;
    bottom: 10px;
    left: 50px;
    text-indent: -9999px;
}