Image

Once you set the src attribute on this image downloading will start, so before that we want to create a handler for the onload event. Once the image has been downloaded this will trigger.

Here’s the complete JavaScript code:

var image = document.images[0];
var downloadingImage = new Image();
downloadingImage.onload = function(){
    image.src = this.src;   
};
downloadingImage.src = "http://an.image/to/aynchrounously/download.jpg";

Caveats of the load event when used with images A common challenge developers attempt to solve using the .load() shortcut is to execute a function when an image (or collection of images) have completely loaded. There are several known caveats with this that should be noted. These are:

• It doesn't work consistently nor reliably cross-browser
• It doesn't fire correctly in WebKit if the image src is set to the same src as before
• It doesn't correctly bubble up the DOM tree
• Can cease to fire for images that already live in the browser's cache

use JQuery plugin Imageloaded