wp-lazy-load

This plugin optimizes your page load performance using the jQuery Lazy
Load. The img src attributes in your posts are replaced with
placeholder tags. The plugin waits for the user to scroll to near
the image before it starts loading the image.

This improves your page speed as only images that are actually viewed by users
are loaded. And your servers will also perform better as they will need
to serve fewer images.

Credits

• Thanks to Mika Tuupola for jQuery lazyload