chromium/chrome/test/data/extensions/api_test/webrequest/csp/img.html

<body>
  <h1 id="result"></h1>
  <script>
    let img = new Image();
    img.src = "./blank.jpg";
    img.onload = () => {
      document.getElementById('result').innerText = 'loaded';
    }
    img.onerror = () => {
      document.getElementById('result').innerText = 'blocked';
    }
    document.body.appendChild(img);
  </script>
</body>