There are various different ways to track down errant album artwork so that your music collection is complete, but one of the easiest, that I have only just come across is 5byfive.  Enter in the name of the album and the artwork pops up.  If it doesn’t get it first time, you can add in other details, band name, etc.  I can’t think of anything simpler.  Haven’t extensively tested it yet, but what I’ve seen so far impresses me.