Icon images loaded by img class

You can do this:

$('.feature-block-item').lazyLoadXT();

However, you need to filter the html for that element through the plugin’s regex before echoing. You can do this by passing it through the get_lazyloadxt_html() function and echoing the response.

See these for more info:
https://github.com/ressio/lazy-load-xt#advanced-initialization
https://wordpress.org/plugins/lazy-load-xt/faq/