CSS image max-width set to original image size?

Just don't set the width of the image, only the max-width.

img {max-width:100%; height:auto}

(height:auto is not really necessary, since auto is the default, but I put it in there as a reminder to myself that I want the image to have its natural proportions.)

This snippet has two boxes, one that is smaller than the image and one that is larger. As you can see, the image in the smaller box gets scaled down, while the one in the bigger box has its normal size.

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.stack.imgur.com/FuQYf.png" alt="" />
</div>
<div class="box2">
    <img src="https://i.stack.imgur.com/FuQYf.png" alt="" />
</div>