I have been known to rip stuff at home, slap the MP3s on a flash drive, and then port them into the iTunes install on the work box. (Which at least partially explains how there are 3600 tracks in the collection at the moment, though I have more like 6000 at home and thousands, maybe tens of thousands, yet unripped.)
It’s a toss up: do you show the CD album cover that the tune belongs to, or do you display an image specific to that song, as though it were a 45 [rpm] single?
If you are trying to sell a full album or EP or maxi-single, then show that image, the cover art.
If you want individual songs to be popular, as the iTune/iPod Generation seems to favor, even not caring who the artist is, just downloading tunes and ringtones based exclusively on the strength of the song itself, which is a revolutionary way of relating to music, then show art specific to each song.
It’s not so revolutionary, really: albums tend to be singles separated by varying amounts of filler. I’ve noticed, though, that iTunes does provide CD-single art for singles, generally, if the album has yet to be released.
Where I draw a lot of blanks is on the singles from the Forties and Fifties, even from the Sixties and Seventies, where the album, had there even been one in the first place, is permanently out of print. And no Beatles or Zeppelin stuff shows up, of course, because iTunes doesn’t vend that sort of thing at the moment. (Of the Beatles solo, I usually get art for George and Ringo, less often John, and hardly ever Paul.) Once in a while, the artwork for a compilation I’ve never heard of will be dropped in simply because the iTunes Store has it. And I’ve seen a few cases where someone somewhere uploaded a scan of a 45 label to Apple, or CDDB, or someplace, and Apple duly sent it to me once I added that track to the playlist. For example: my rip of “Come to Me Softly” by Jimmy James and the Vagabonds yielded up a scan of the Atco 45; as a test, I purchased a copy of the same song from iTunes, which gave me the artwork for the compilation Sock It To ‘Em J.J.: The Soul Years, a UK release I haven’t yet seen Stateside.