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>
For background images use .lazy-bg class.
<div class="lazy-bg" data-src="img/assets/projects/bg-5.jpg"></div>