These instructions are based on my Windows version of iTunes. YMMV
Select all the songs that you want to change the cover art for (or want to apply the same art to), right-click one of them and select "get info", and go to the artwork tab. From there you can cycle through the cover art embedded in the song/songs you selected. Select any of them that you don't want and hit delete. Once you've done that you can hit add and import the cover art you want. This of course assumes that you have the art that you want to use on your computer already.
Hope this helped. As I said, these instructions may or may not quite apply to a Mac (although I think you can still get to the "Get info" from the File menu).