image-decode-promise.js

const img = new Image();
img.src = "cat.png";
img.decode().then(() => {
  // image fully decoded and can be safely rendered on the screen
  const orig = document.getElementById("orig");
  orig.parentElement.replaceChild(img, orig);
});

image-decoding.html

<!-- Suggest to the browser that the decoding can be deferred -->
<img decoding="async" src="cat.png" >

<!-- Suggest to the browser that the decoding should not be deferred -->
<img decoding="sync" src="cat.png">
                                  
<!-- Let the browser figure it out -->
<img decoding="auto" src="cat.png">
<img src="cat.png">