First ensure that you are using the JMS 1.1.17 that fix a problem in the administration to allow using the deployed directory for the administration instead of the master directory.
This version allow tools like the media manager to access the deployed directory.
With previous JMS version (<1.1.17) the access to the deployed directory is only available from the front-end (not the back-end).
Go to
www.jms2win.com/get-latest-version to receive a new download ID corresponding to the latest version of JMS.
Second, when you create the slave site in JMS and that you specify the image and media folder, this result by updating the new website DB by a modification of the Global Configuration / Server / Media settings.
This also result by a copy of the image & media folder into the new path specified.
When nothing is specified, JMS creates a Symbolic Link in the deploy directory.