An audio CD doesn't contain any artwork, or indeed much in the way of metadata at all. All the iTunes normally sees is the pattern of track lengths. This is used to recognize the CD in the Gracenotes CDDB database. Having fetched the track details, songs names, artist, and album title, it may then use a separate mechanism to match this album in the iTunes Store catalog so it can fetch artwork. I've not personally ripped an album in ages, so I cannot recall how successful this process usually is. Before ripping the CD you can try right-clicking the album and using Get Album Artwork if it doesn't arrive automatically. iTunes has two methods for associating art, it can cache an image from the store, or it can embed artwork in the tag for formats other than .wav. When ripping a CD I think it uses the cached download rather than embedding. One would expect all tracks from the same album to be treated equally, but iTunes doesn't always manage this and can hold an album internally in two or more groupings. In such cases one grouping might be assigned art but another not. For consistency I will generally re-embed the artwork for an album in all tracks after I've imported it to the library after fixing any grouping issues.
See Grouping tracks into albums for a general discussion on the topic of split albums.
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