Using a medium from a library
In the object tree editor, select the medium node that you would like to replace by a library medium.
Right-click on it, and change its type to linked.
In the node properties pane, edit the target parameter text box. Then select a medium in the libraries toolbox, then drag and drop it in the <target> box.
You can also type the target manually in the form : library_name::medium_name
Click on "Apply" to save your linked medium