PageSpeed Insights doesn't recognize WebP format when using jpg fallback

I'm trying to get rid of the message "Serve Images in Next-Gen Formats" in PageSpeed Insights (PSI). In order to do that, I'm changing all HTML code of the images to:

<picture>
  <source type="image/webp" srcset="imgwebpversion.webp">
  <source type="image/jpeg" srcset="imgjpegversion.jpg">
  <img src="imgjpegversion.jpg" alt="alt description here" width="200" height="56">
</picture>

But PSI keeps on showing the message. Does anyone know what's the correct approach to serve webp images and still support old browsers?

Thank you very much for your time.