Add lazy.js to the footer of your site just below jquery.
Lazy load <img> or background-images on page scroll.
In your HTML image tag just add data-image e.g:
<img src="lowres.jpg" data-image="mainimage.jpg" />
and using inline background-image style just add data-background:
<div style="background-image: url('lowres.jpg');" data-background="mainimage.jpg"></div>