To ensure maximum performance of your page all the images (including backgrounds) are loaded in deferred way. It speeds up page loading times and decreases traffic to your users by only loading the content in view.
To take advantage of this feature you should use special markup for your images. Instead of putting image source URL to standard [src] attribute, put it in [data-src] like in the following example:
<div class="lazy"><img data-src="img/assets/project5/img-5-3.jpg" width="960" height="960" src="#" alt=""/></div>
To minimize page reflow events, it’s recommended that you always specify width and height of the image. Using the dimensions the script will calculate the correct wrapper width and height so your images will not jump when they are being loaded.
For background images use .lazy-bg class.
<div class="lazy-bg" data-src="img/assets/projects/bg-5.jpg"></div>