Here:
var image = new Image();
image.src = "https://webglfundamentals.org/webgl/resources/leaves.jpg"; // MUST BE SAME DOMAIN!!!

This is incorrect. You can use:
var image = new Image();
image.crossOrigin = "*" // or crossOrigin = "anonymous"
image.src = "https://picsum.photos/300/500"; // try it, this works!

The only caveat is that credentials are omitted, but I think that's acceptable.