Modify WordPress img src for the “get_the_post_thumbnail”

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Expose the WordPressimg 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"

Related Links:
Source: http://chono.com/notes/modify-wordpress-img-src-get_the_post_thumbnail