HTML
<picture>
<source media="(min-width: 40em)" srcset="//placehold.it/250x100/fff 250w, //placehold.it/400x166/fff 400w, //placehold.it/600x250/fff 600w, //placehold.it/1000x400/fff 1000w" alt="portfolio thing" />
<img sizes="(min-width: 40em) 90vw, 100vw" srcset="//placehold.it/250x125/fff 250w, //placehold.it/400x200/fff 400w, //placehold.it/600x300/fff 600w" alt="portfolio thing" />
</picture>