This 700 bytes (compressed) library enhances Picture Elements with Progressive Image Loading and thus improves the initial time of images being displayed. You can find out more from the blog post.

$ npm install progressive-picture

<script type="module" src="https://unpkg.com/progressive-picture"></script>

picture > img {
  display: block;
  max-width: 100%;
  height: auto;
  filter: blur(0);
  transition: filter 0.7s ease-in;
}
picture > img[data-src] {
  filter: blur(3px);
}
picture > img.img-progressive {
  width: auto;
}

<picture>
  <source srcset="eu.preview.webp" data-src="eu.webp" type="image/webp" />
  <source srcset="eu.preview.jpg" data-src="eu.jpg" type="image/jpg" />
  <img
    src="eu.preview.jpg"
    data-src="eu.jpg"
    loading="lazy"
    width="500"
    height="750"
    alt=""
    data-alt="Eukalyptus"
  />
</picture>

https://lazy-load-picture.netlify.app/
https://lazy-load-picture.netlify.app/masonry.html

The srcset of <source> and the src of <img> has to be filled with the URI for the low-quality image (or in case of the <source> element: a srcset of low quality preview pictures. The data-src holds the high-quality image(s) and will be replaced once it has been loaded.
Also, there is a data-alt Attribute that can be applied as alt, once the Image was replaced. This fixes the inelegance of displaying text before an image appears.

While this library is useful on the Web, optimizing the image correctly in the build step is the other side of the coin. This plugin snowpack-plugin-sharp could be helpful in this case.