You can move the uploads folder to the sub domain by doing this
Open up your wp-config.php file, located at the root of your WordPress installation, and add the following code:
define('UPLOADS', 'http://images.mydomain.com/uploads');
The codex specifies that it should be added before the line that says require_once(ABSPATH.’wp-settings.php’);.
Make sure the uploads folder is writable.