<!-- Markup -->
<div class="container">
    <div class="imageOne image"></div>
    <div class="imageTwo image"></div>
</div>

<!-- CSS(important to add height to container class as well as image class since we have empty divs) -->

.container {
    position: relative;
    height: 100px;
}
.image {
    position: absolute;
    width: 100px;
    height: 100px;
    border: 3px solid black;
}

.imageOne {
    z-index: 0;
}

.imageTwo {
    z-index: 1;
}