chromium/chrome/test/data/load_image/two_images.html

<html>
<head></head>
<img/>
<img/>
<script src="common.js"></script>
<script>
function checkBothImagesLoaded() {
  return new Promise((resolve, reject) => {
    Promise.all([imageLoadedPromise(document.images[0]),
      imageLoadedPromise(document.images[1])]).then(() => {
        resolve(true);
      });
   });
}

window.onload = () => {
  setTimeout(() => {
    document.images[0].src = "image.png"
    document.images[1].src = "image.png?foo"
  }, 1000)
}
</script>
</html>