Change filename on upload for jpg and png files.

hash-upload-filename.php

<?php

/**
 * Plugin Name: Hash Upload Filename
 * Plugin URI:  https://stackoverflow.com/questions/3259696/rename-files-during-upload-within-wordpress-backend
 * Description: Rename uploaded jpg and png files as the hash of their original. Extension remains same.
 * Author: Web Savvy Marketing
 * Author URI: http://www.web-savvy-marketing.com
 * Version:     1.0
 */

/**
 * Filter {@see wp_handle_upload_prefilter } and return an MD5 hash.
 *
 * @param  string $filename
 * @return string
 */


function wsm_wp_modify_uploaded_file_names($file) {
    $info = pathinfo($file['name']);
    $ext  = strtolower( empty($info['extension']) ? '' : '.' . $info['extension'] );
    $name = basename($file['name'], $ext);

    if ( $ext == '.jpg' || $ext == '.png' ) $file['name'] = md5($name) . $ext; // md5 method

	return $file;
}

add_filter( 'wp_handle_upload_prefilter', 'wsm_wp_modify_uploaded_file_names', 1, 1 );