Resize images that are NOT attachments [WordPress]

resize_images_gd_image_editor.php

/**
* In this example book covers are stored in custom fields. 
* The max size used on site for book cover 300px *450px, so there is no point to store image large than this. 
* We'll resize all existing book covers to this size using GD library. 
* This is popular PHP image processor widely supported by hosting providers out of box. 
* Also WordPress itself has got in-built support for that through WP_Image_Editor_GD class. Great for development!
**/

require_once( ABSPATH . 'wp-includes/class-wp-image-editor.php' );
require_once( ABSPATH . 'wp-includes/class-wp-image-editor-gd.php' );
//Get all bookcovers
$args = array( 'post_type' =>'post', 'posts_per_page' => -1, 'post_status' => 'any', 'fields' =>'ids');
$myposts = get_posts( $args ); 
	foreach ($myposts as $mypostid) {  
		$bookcover = get_field('bookcover', $mypostid);
		//Image options for resize
		$max_w = 300;
		$max_h = 450;
		$crop = false;
			$uploads_dir = wp_upload_dir(); 
			$replace = $uploads_dir['basedir'];
			$search = $uploads_dir['baseurl'];		
				//We need path here not url
				$image_path = str_replace($search, $replace, $oblojka_knigi);
				$gd_image_editor = new WP_Image_Editor_GD($image_path);			
				$gd_image_editor->load();  
				$current_size = $gd_image_editor->get_size();	
					//Proceed only with images that are larger than desired size
					if (($current_size[width] > $max_w) || ($current_size[height] > $max_h)){			
					$gd_image_editor->resize( $max_w,$max_h,$crop); 
					$result = $gd_image_editor->save($image_path);  
					//print_r($result);	 	 
					}			
	}