Dark mode images without JavaScript

You do not need JavaScript to swap an image for dark mode. The <picture> element accepts media queries on its <source> elements, including prefers-color-scheme.

<picture>
  <source srcset="dark.png" media="(prefers-color-scheme: dark)">
  <img src="light.png" alt="">
</picture>

When the visitor prefers dark mode, the browser uses dark.png. Otherwise it falls back to light.png from the <img> element.

Use a useful alt value when the image carries meaning. I left it empty here because the example is only about choosing the source.