Sometimes iTunes messes up on the covers. Its pretty good if its a big name or common album. But some older stuff and or music not hugely popular can have a low res or no cover at all. But never fear, if you can google an image up or any image. You can choose the album cover in iTunes by selecting the album or the songs and then Right Click and choose Get Info, from there it will ask if you want to edit multiple tracks, choose "Yes". Now double click in the white square under Artwork and the locate the image you want and then press Open. Make sure you have the CheckBox next to the Artwork clicked and then choose OK. Thats it..
I tend to have this issue with my Bjork albums, mostly the rare single stuff or alternate albums of hers. Also strangely enough one of my Beastie Boys albums doesn't show up and a few of some Dance/Techno albums don't show up or show crazy artwork not related to the album. All of these a purchased the past 12 or 15 years.