Can you provide the details of how you added the Media? It may help to explain why the cover art is longer visible.
BTW, here's how Premise maintains a Media library:
I'm assuming you created a File Provider and pointed it to a folder containing all of your music and cover art. The File Provider builds an index of everything it discovers in the folder (and sub-folders). Premise keeps the index updated by monitoring the folder for additions and deletions. Premise maintains an index and does not retain a copy of any media. If the media file (audio, video, images) is deleted from the monitored folder, it will disappear from the index as well (i.e. it will no longer be visible in Premise). Premise maintains the index in its data-store, in memory, and writes it out periodically to disk in XML format.