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

<html>
<body>
<img alt="long_placeholder_text"/>
<script src="common.js"></script>
<script>
window.onload = () => {
  setTimeout(() => {
    document.images[0].src = "private_url_image.png"
  }, 1000)
}
</script>
</body>
</html>