Did you change either the "Album Artist" field or the "Compilation" setting?
To try and replicate the problem, I took one compilation (which had the "Compilation" tag set and nothing listed in the "Album Artist" fields) and added completely different artwork for each track. When sorted by album, it showed one piece of artwork as usual, from the first track. However, when not sorted by album, iTunes still showed the artwork from the first track for each song.
When I unckecked the "Compilation" tag, the songs showed their individual artwork, and were organized as twelve different albums, each with the same name.
When I added the name "Various Artists" to the "Album Artist" field, once again they showed the first cover for all tracks.
It looks like you can either set the "Compilation" tag to make iTunes think it's all one album, or you can make sure the "Album Artist" and "Album Title" fields so they are the same for all tracks. Differing artwork doesn't seem to come into it either way.
I usually use the "Compilation" tag, and ignore the "Album Artist" tag on compilations, unless the album is a mix done by a DJ, or a Greatest Hits compilation that includes tracks from two or more different acts.
For instance, the album "Osymyso Brings You... The Art Of Flipping Channels" is tracks by various artists that have been mixed together by the DJ Osymyso. So each track has it's own "Artist", but the "Album Artist" field for all tracks is "Osymyso".
Similarly, "The Pretenders Greatest Hits" includes tracks by "The Pretenders" (of course), as well as tracks by "Moodswings & Chrissie Hynde" and "UB40 & Chrissie Hynde". In this case the "Album Artist" is "The Pretenders".
The albums are then neatly listed as compilations under "Osymyso" and "The Pretenders" respectively.
PowerMac Dual 1.42 GHz G4 Mac OS X (10.4.7)