The guide for this code is available at http://extracatchy.net/resize-wordpress-images-on-upload

functions.php

<?php

/**resize on upload to the largest size in media setting */

function replace_uploaded_image($image_data) {
// if there is no large image : return
if (!isset($image_data['sizes']['large'])) return $image_data;

// path to the uploaded image and the large image
$upload_dir = wp_upload_dir();
$uploaded_image_location = $upload_dir['basedir'] . '/' .$image_data['file'];
$large_image_location = $upload_dir['path'] . '/'.$image_data['sizes']['large']['file'];

// delete the uploaded image
unlink($uploaded_image_location);

// rename the large image
rename($large_image_location,$uploaded_image_location);

// update image metadata and return them
$image_data['width'] = $image_data['sizes']['large']['width'];
$image_data['height'] = $image_data['sizes']['large']['height'];
unset($image_data['sizes']['large']);

return $image_data;
}
add_filter('wp_generate_attachment_metadata','replace_uploaded_image');

How do you use that code while keeping the original uploaded image?