Create/Load a default post thumbnail (WordPress theme)

default-post-thumb.php

<?php

/**
 * Default Img for post thumbnails
 *
 * @param $file_name_new  (string) Name of the newly-created file
 * @param $file_name_orig (string) Filename of the original default image
 * @param $post_id   (int)    ID of the associated Post
 * @param $size      (array)  Width/height of cropped thumbnail
 * @param $class     (string) Optional class attribute for output image
 * @param $show_link (bool)   Truthy wraps IMG in permalink. Falsy does not.
 */

function get_default_post_thumbnail( $file_name_new = '', $file_name_orig = '', $post_id, $size, $class = '', $show_link = false ) {

    $class = ('' != $class) ? 'class="' . $class . ' "': '';

    if( '' === $file_name_new )
        $file_name_new = 'img_default_thumb-';

    if( '' === $file_name_orig )
        $file_name_orig = 'img_default_thumb.png';

    $thumb_path = get_stylesheet_directory() . '/img/'.$file_name_new . $size[0] . 'x' . $size[1] . '.png';

    // check if thumbnail already exists
    if ( !file_exists( $thumb_path ) ) {
        $img_path = get_stylesheet_directory() . '/img/' . $file_name_orig;

        if( file_exists( $img_path ) ) {
            $image = wp_get_image_editor( $img_path );
            if ( ! is_wp_error( $image ) ) {
                 $image->resize( $size[0], $size[1], true );
                 $image->save( $thumb_path );
                 $thumb_img = basename($thumb_path);
            }
        }
    } else {
        $thumb_img = basename($thumb_path);
    }

    $html = '<img ' . $class . ' src="' . get_stylesheet_directory_uri() . '/img/' . $thumb_img . '" />';

    if ($show_link) {
        $html = '<a href="' . get_permalink( $post_id ) . '" title="' . esc_attr( get_post_field( 'post_title', $post_id ) ) . '">' . $html . '</a>';
    }

    return $html;
}