How to store media files in subdomain

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.