I think I just found the answer to my own question. The <picture> tag looks something like:
Code:
<picture>
  <source type="image/webp" srcset="flower.webp">
  <source type="image/jpeg" srcset="flower.jpg">
  <img src="flower.jpg" alt="">
</picture>
In the event that the <picture> element is not recognized by the browser (like IE), everything is ignored except for the <img> tag. So the image stills shows up. (Source: https://web.dev/serve-images-webp/). This will be very useful.