Lazy loader is a jQuery plugin written in JavaScript. It delays loading of images in (long) web pages. Images outside of viewport (visible part of web page) wont be loaded before user scrolls to them. This is opposite of image preloading.

Using lazy load on long web pages containing many large images makes the page load faster. Browser will be in ready state after loading visible images. In some cases it can also help to reduce server load.

via Lazy Load Plugin for jQuery.

There is also a WordPress plugin.