Where did this album artwork come from???????

Recently I sync'd my iPod (5G) and iTunes added an album cover to all my songs in the Rock genre. All my albums in the rock genre now have the same artwork on my iPod. However on my iTunes, the album artwork is correct for each bands individual album and there is only one album cover in each songs information file. How did this happen? This is the first time this has ever happened. There are even a couple of random albums I bought on iTunes where the album cover art was placed on some bands album art which have not been changed in a year, but on iTunes the correct album cover is the one shown on my computer screen. Am I making any sense? Can you guys help me??

Dell Windows XP