That technique was used in older browsers with luck, but now modern browsers are not longer cheated with this way, so you have to do blank src and data source attribute on server side, here is popular jQuery lazy load plugin homepage, http://www.appelsiini.net/projects/lazyload you bet they did a lot of researches before writing this
Latest version of Lazy Load is not a drop in replacement to your webpage. New browsers load image even if you remove the src attribute with JavaScript. Now you must alter your html code. Put placeholder image into src attribute of your img tag. Real image url should be stored data-original attribute.
UPD: just think over a bit and probably better approach to data- attribute would be using noscirpt
wrapping for images you do want loaded lazlily here roughly illustration for idea http://jsbin.com/uqomeb/2/edit I would possibly do simple jQuery plugin later based on this