Edit in JSFiddle

body {
    overflow: hidden;
    margin: 0;
}

.container {
    position: relative;
    max-width: 600px;
    margin: 0 auto;
}

img {
    width: 100%;
}

.caption {
    position: absolute;
    display: block;
    bottom: 0;
    left: 0;
    right: 0;
    padding: 15px;
    text-align: center;
    background: #000;
    background: rgba(0, 0, 0, .8);
    color: #fff;
    font: 14px/24px sans-serif;
    margin-bottom: 0;
}