restrict to subdirectory not work and media module load all images in public://
Option 1
Media Browser Plus module states that it will allow

Personal and public “folders”

Which might answer to your needs
Option 2
Views Media Browser should allow you to specify a view to use as the media browser - you could then filter the view based on file path or other file attribute to get the result you are after.
Option 3
If you are using v2 of Media module you can apparently define a View with a 'views media browser' display to create your media module listing. Media module 2.x is currently in alpha, so take care if you wanted to go down this route, and obviously do not test on live site.
see https://drupal.org/node/1547884