From Georgetown University Commons Knowledge Base
Your Media Library holds all of the files that you have uploaded. You can link and re-link to these files, allowing you to refer to the same file multiple times on your blog.
Managing your media library
You can add files to your media library by clicking on "add new" under "media library." This will simply add files to your library; it will not attach them to any page or post.
To remove files, hover over the title of the file and select the "Delete Permanently" option that appears.
To attach a file (anything but a photo) to a page or post, click on the title of the file. You will see a "file URL" box in the right side file information menu. Copy this URL to your clipboard. You can then create a hyperlink in your post or page to this URL, and, when clicked, it will open the file in the appropriate application.
To insert a photo that you already have in your media library into a post or page, go to that post or page and click on the "Add Media" button above the editing box. Select "Media Library" from the tab options at the top. All photos in your Media Library should appear. Click the photo you want to insert; a blue box will appear around it. Click "Insert into post" in the bottom right corner.
If you are inserting a text link into a document or other file, don't worry if you cannot click and open the document from dashboard. The link will only be active on the public side of the blog. You will know your link was successful if the text from which you were creating a link is blue/underlined.