The url read-only property of the PerformanceElementTiming interface returns the initial URL of the resource request when the element is an image.

A string which is the initial URL of the resources request for images or 0 for text.

In this example an <img> element is being observed by adding the elementtiming attribute. A PerformanceObserver is registered to get all performance entries of type "element" and the buffered flag is used to access data from before observer creation. Calling entry.url returns https://example.com/image.jpg.

<img
  src="https://example.com/image.jpg"
  alt="a nice image"
  elementtiming="big-image"
  id="myImage" />

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    if (entry.identifier === "big-image") {
      console.log(entry.url);
    }
  });
});
observer.observe({ type: "element", buffered: true });