Thank you for your help. It seems that there is no hubl way to add lazy load to its images and I think the only option is use a js library. And I've decided to use lozad.js.
How to implement lazy loading in images to web pages?
if you use AMP dont use these lazy load images like lozad on blog post images. Or the Hubspot AMP parser will remove all your images on AMP pages.
This happens due when converting <img> tags to <amp-img> Hubspot will strip completely all tags that dont have a valid src property. Also it will strip and data-src property. So no images will be displayed on AMP pages.