MC will look-up track info and cover art from JRiver's own own database called YADB. It will then look in FreeDB. If nothing is found, you can go copy an image (either by file, or by clipboard) and apply that image to your tracks.
Go find your ripped tracks, select them, and right-click and select Cover Art->... at the bottom of the pop-up menu. Add the cover art by Pasting (if in the clipboard) or by Add from file (if you have an image file to use as the artwork).
Artwork look-up is enabled/disabled in Options. Also, so is where you save the artwork (i.e. which folder; within the tracks themselves).
See Cover Art in the Wiki:
http://wiki.jriver.com/index.php/Cover_Art
