How to Modify WordPress “img src” for the “get_the_post_thumbnail”

Expose the WordPress “img src” part from the get_the_post_thumbnail, to modify the “img src” syntax.

Instructions
Add the following code to your functions.php file of the theme you wish to use the custom image sizes for.

<?php
   $imgs = get_the_post_thumbnail($post->ID);
   /*get the img URL, delete the <img /> tag */    
   $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $imgs, $matches);
   /*the image URL*/
   $img = $matches [1] [0];
   /*echo the full img*/
   echo '<img src="' . $img . '" alt="" />';
?>

*note, for adding complimentary markup, classes and ID’s, you can do so by using regular expressions such as class="img-responsive"