chromium/chrome/browser/page_load_metrics/integration_tests/data/full_size_image.html

<script>
  const lcpPromise = new Promise(resolve => {
    window.addEventListener("message", event => {
      iframeLCP = event.data.startTime;
      timeOriginDelta = event.data.timeOrigin - performance.timeOrigin;
      resolve(iframeLCP + timeOriginDelta);
    });
  });
  async function waitForLCP() {
    return lcpPromise;
  }
</script>
<iframe width="96" height="96" src="iframe_with_image.html"></iframe>