image.onload event and browser cache

As you’re generating the image dynamically, set the onload property before the src.

var img = new Image();
img.onload = function () {
   alert("image is loaded");
}
img.src = "img.jpg";

Fiddle – tested on latest Firefox and Chrome releases.

You can also use the answer in this post, which I adapted for a single dynamically generated image:

var img = new Image();
// 'load' event
$(img).on('load', function() {
  alert("image is loaded");
});
img.src = "img.jpg";

Fiddle