Hi,
As far as I know, there are two ways to replace cover art for every song within an ablum in iTunes.
1) Go to View-show brower
2) Go to View-album view
3) Select your album in your top right brower (under Album)
4)Go to File-Get info
Pops up appears: "Are you sure you want to edit information for multiple items?"
click " yes"
5) "Multiple item information" window pops up,
copy and paste your album cover art to the Artwork square and click "OK".
6)Pop up appears: 'Are you sure you want to change the artwork for multiple items?"
click "Yes".
The new cover art should attach to every song in the album.
Another way to do it seems to be a little faster-
repeat step 1 to 3
4)Now select one of the songs in the song list.
5)Go to File-Get info
A different looking window will pop up.
6) Select the "Artwork" tab.
If you see the wrong cover now, right click and delete it.
7) Copy and paste the correct album cover to the empty square and click "OK"
Done!
Amazon is still the best place to get decent album cover. Or you can paste any images you want into itunes( I did that sometimes when a "best of" cd has a badly done cover and I would rather have the artist's photo there.)
Hope this helps.
-Paul