You may have being on other sites that when you scroll down the page, photos load as your scrolling, so where you are on the webpage the photos are showing, but where you are not on the webpage the photos have not loaded.
This speeds up your WordPress site loading, as only the photos towers the top of the page load first.
This is very simple done with a plugin called Lazy Load. Just install it from WordPress.Org and activate. You should notice an increase in your loading speed especially if you have lots of photos.
This plugin was developed by an amalgamation of code written by a few developers including WordPress.com VIP team.
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