itunes
I change the cover images for the songs in my iTunes library using:
Right Click > Get Info > Artwork > Drag-Drop > OK
and in most cases the artwork updates immediately. On some tracks, though, it doesn't.
I then tried removing the artwork (so nothing should appear in Cover Flow), but the track then keeps the image it had previously.
In iTunes > Preferences > Advanced
Uncheck Keep iTunes Media folder organised and Copy Files to iTunes Media folder when adding to Library
Screen shot provided by @Wheat Williams:
You may not be able to get complete matches, but here's what I have been doing in the cases where one or two tracks don't match.
Note that this will keep the play counts, etc on your tracks. It may not succeed on every track, and sometimes repeating the process picks up another track.
Occasionally, I'll get a "Removed" icon there after doing this: just right-click and choose "Add to iCloud" on these.
Best Answer
iTunes holds its data in an XML structure. On the surface what may appear to you to be a single Album file consisting of 10 separate tracks may, in fact, be 10 separate tracks, each with a separate link to the Album artwork. How this comes about I am not sure, but I think it's related to how the files were input to iTunes. For example, ripping your own CD may produce a different result from 'acquiring' the same album from - shall we say, a download sharing site.
Normally what I do is
(1) if an album has no artwork, I select all the tracks and drag'n'drop the artwork (from Amazon for example) into the box for Selected Item. Normally this will lock the artwork to EVERY track in the album. If, however, the structure isn't 'correct' the artwork will only be locked to a single track. You can check by clicking on several tracks in the same album.
(2)If the artwork isn't there for all the tracks, you can select all the tracks, then go to File>Get Info and drop the artwork into the Artwork Box. You should then have artwork for every track
(3)Failing all this, as a last resort I drop the artwork in for each track separately.
One of these methods always works for me.
Bit tedious for a few albums, but for more than 1500 albums in my Library, I've only had to go the most tedious route a few times.
I should add that iTunes allows you to include several versions of the artwork for each track. If you replace artwork you may in fact simply be adding another version. Which one gets displayed is determined in the following way. Click a track. Go to File>Get Info>Artwork... here you will see all the versions of the artwork. Delete the ones you don't want. The one you are left with is the one that gets displayed.
I know.. it's confusing..