How to put code in Loop

Hello. Please help me because I keep looking and looking but can’t find a solution since I’m new to WordPress. I need to have a plugin’s code inside the theme’s loop but I don’t know where exactly I should place it.
It’s a plugin for displaying featured images in posts. Until now I put manually the shortcode before each post but I want it to be automated. The creator of the plugin says this code must be placed inside Theme Loop:

<?php if ( function_exists(‘get_featured_img’) ) get_featured_img(); ?>

I hope you could help me!
Thanks.