AJAX: fading in images

I've noticed the images on mashable.com fade in as I scroll down the page. I guess they are loaded with AJAX to reduce the page weight in the beginning.

Is there a jquery plugin for doing this? What's the best way to add this feature to my website开发者_如何学编程?

thanks

---

It's called lazy loading of images. There's a jQuery plugin here. There are probably many more if you have a look around...