I did this few months before after seeing an article on WP Beginner. I have made some security changes via .htaccess file and switched over to TwentyFourteen. My subdomain folder is Media and I can upload images into this directory whenever I try to upload.
The problem is that on live site it doesn’t show media.mydomain.com rather than _mydomain.com/media/2014/01/download_gesture_icons-672x372.jpg_ for an example via this post http://goo.gl/j1YUBV, you can see them by right clicking on the first image and inspect.
Note This is happening only with my latest 4-5 articles as older published images show http://www.media.mydomain.com
What do you think! what went wrong or overwritten.