Allows you to override WordPress' auto-generated images

ImageOverride

<?php
/*
Plugin Name: Image Override
Plugin URI: http://www.billerickson.net
Description: Allows you to override image sizes
Version: 1.0
Author: Bill Erickson
Author URI: http://www.billerickson.net
License: GPLv2
*/

class BE_Image_Override {
	var $instance;
	
	public function __construct() {
		$this->instance =& $this;
		add_action( 'init', array( $this, 'init' ) );	
	}
	
	public function init() {
		add_filter( 'cmb_meta_boxes', array( $this, 'create_metaboxes' ) );
		add_action( 'init', array( $this, 'initialize_cmb_meta_boxes' ), 50 );
		add_action( 'image_override_display', array( $this, 'display' ), 10, 1 );
		add_action( 'return_image_override_display', array( $this, 'return_display' ), 10, 2 );
	}

	public function create_metaboxes( $meta_boxes ) {
		
		$post_types = apply_filters( 'image-override-post-types', array_keys( get_post_types( array('show_ui' => true ) ) ) );
		$sizes = apply_filters( 'image-override-sizes', array( 'thumbnail', 'medium', 'large' ) );
		
		if ( !empty( $sizes ) ) {
			
			$prefix = 'image_override_';
			$fields = array();
			foreach( $sizes as $size ) {
			
				$fields[] = array(
					'name' => ucwords( $size ), 
					'desc' => '',
		            'id' => $prefix.$size,
		            'type' => 'file'			
				);
	
			}
		
			$meta_boxes[] = array(
		    	'id' => 'image-override',
			    'title' => 'Image Override',
			    'pages' => $post_types,
				'context' => 'normal',
				'priority' => 'low',
				'show_names' => true, 
				'fields' => $fields
			);
		}
		
		return $meta_boxes;
	}

	public function initialize_cmb_meta_boxes() {
		$sizes = apply_filters( 'image-override-sizes', array( 'thumbnail', 'medium', 'large' ) );
	    if ( !class_exists('cmb_Meta_Box') && !empty( $sizes ) ) {
	        require_once( dirname( __FILE__) . '/Custom-Metaboxes-and-Fields-for-WordPress/init.php' );
	    }
	}
	
	public function display( $size = '' ) {
		global $post;
		echo $this->return_display( $post->ID, $size );
	}
	
	public function return_display( $id = '', $size = '' ) { 
		$override = get_post_meta( $id, 'image_override_'.$size, true );
		if ( !empty( $override ) ) return '<img src="' . esc_url( $override ) . '" class="attachment-'. esc_attr( $size ) . '" alt="' . get_the_title($id) . '" title="' . get_the_title($id) .'" />';
		else return get_the_post_thumbnail( $id, $size );	
	}

}

new BE_Image_Override;
?>

Uses this custom metabox code: https://github.com/jaredatch/Custom-Metaboxes-and-Fields-for-WordPress