Wrong Album pictures showing

Using Windows Media Player 11 to rip a pile of albums to put on wife's new
Walkman. Occasionally it can't find the correct album cover or track info so,
after a search or edit it is saved with the plain silver cd showing. But when
you look in the Library an album cover from one of the other cd's has been
added.
Anyone know why it does this and can wrong album covers be deleted?
Thanks.