In this article, Patrick Sexton explains how to defer certain images without JQuery for faster page loading. I applied this technique to my category/subcategory images and I'm very happy with the result. I noticed my website loads much faster.
https://varvy.com/pagespeed/defer-images.html These were the two files I have used:
/storefront/view/default/javascript/common.js
/storefront/view/default/template/blocks/category-top.tpl
NOTE: Along with image deferment, I also used image compression.