responsive image

layout.css

.box1 {
    width: 100px;
    height: 200px;
}
.box2 {
    width: 300px;
    height: 500px;
}
div {
    position: relative;
    background-color: #000;
    margin-bottom: 30px;
}
div img {
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;    
    margin: auto;
    width: auto;
    max-width: 100%;
    height: auto;
    max-height: 100%;
}

sample.html

<div class="box1"><img src="http://placehold.it/300x250" alt="" /></div>
<div class="box2"><img src="http://placehold.it/300x250" alt="" /></div>