Aggiungere semplicemente il seguente codice nel file functions.php del proprio tema.

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

    // paths 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');