I have had to go through iTunes at least 3 times to change artwork that was incorrect on some albums.
The easiest thing I found to do was to go to a site where they have artwork for albums, I like to use Amazon, as they usually have the artwork I am looking for. Copy the artwork image to the clipboard by right clicking and choosing copy.
Then go into iTunes and view your albums in cover view. Right click on the artwork and choose get info. When the info box opens, it will be for the entire album (or the few songs) and in the Album art box, paste the copied artwork (right click in Windows and choose paste, for a mac, choose the edit function in the menubar, and paste). The checkbox next to the box should fill and then click save. This will change the artwork for all of the songs in the folder at the same time.
One thing I did notice when I was using Windows Media Player (before I bought my Macbook) the album art is saved in the album folders. They are a hidden file.
If you are on a Windows machine, go to folder options and choose the option to view hidden and system files. The artwork files should appear the next time you open the folder that the music files are in. Make sure you are in thumbnail view when in the folder so you can see the artwork. Delete the wrong files (sometimes there are up to 3 or more) and paste the new artwork again, as previously mentioned. This may solve the problem.