Example
Resize the viewport.
HTML
<picture>
<source media="(min-width: 1600px)" srcset="images/AlleySpringMilllarge.jpg">
<source media="(min-width: 800px)" srcset="images/AlleySpringMillmedium.jpg">
<source media="(min-width: 400px)" srcset="images/AlleySpringMillsmall.jpg">
<img src="images/AlleySpringMillsmall.jpg" alt="fallback">
</picture>
CSS
img {max-width: 100%;
height: auto;
}