If iTunes gets the wrong artwork, or sometimes it can't find the artwork, another way to do it is to search for the particular CD in an on line retailer such as amazon. When you find the CD in amazon select it and it will then usually show a picture of the album art in the left hand corner. Right click on the album art and select copy. The go back to iTunes and select all of the tracks for the album with the missing artwork and right click and select Get Info. I will then ask are you sure that you want to change info for all selected tracks, just click OK. It will then bring up a box called Multiple Item Information and you will see on the right hand side towards the bottom a box called artwork. Move the cursor over the Artwork box and right click, then selcet paste. You should now see you correct Artwork in this box, then click OK and it adds the correct artwork to all of the selected tracks.
Hopefully this should work for you.