This is a problem that’s been there from the beginning. It’s not iTunes fault, it relies on Gracenote for track info, and generally Amazon for cover art. Amazon don’t always match the cover art properly, and Gracenote don’t always get the track info right either. There is software available, called Coverscout, that aims to get all the right cover art, but I’ve never used it, so I don’t know how effective it is.
As said its easy to sort, just select the top track for the album, shift click on the last track to select them all, then drag the largest image file you find into the artwork window at the bottom.
I forgot to check the ‘find artwork’ button when I got my new Mac Mini, so I’ve got a couple of dozen albums lent by a mate that I had to find the cover art for. PITA!
My fault, though. And I still find albums that have incorrect artwork on, out of 8-900 in iTunes.
I might see about getting Coverscout myself, if I can find some reviews.