div {border:2px outset green; margin:6px 0}
.box1 {width:100px; height:70px;}
.box2 {width:200px; height:100px;}
img {max-width:100%; height:auto}

<div class="box1">
    <img src="https://i.sstatic.net/FuQYf.png" alt="" />
</div>
<div class="">
    <img src="https://i.sstatic.net/FuQYf.png" alt="" />
</div>