iTunes will normally display album artwork as long as at least one track has embedded or store associated artwork. Adding artwork to one track doesn't automatically embed it any others from the same album.
I have two scripts that can help with embedding artwork; CreateFolderArt and EmbedFolderArt. The scripts have slightly different functions:
CreateFolderArt ensures that every album folder ends up with a Folder.jpg image which is the art that iTunes already knows about. Side effects are that if artwork has been updated in iTunes the folder art should be updated also, and if any track from the album doesn't have embedded artwork it gets embedded.
EmbedFolderArt was actually written for someone who already had various artwork images stored in the album folders and wanted them added to their tracks, but not if iTunes had already downloaded a better quality image. It creates new files of any store art with the name iTunesArt.jpg, then embeds the largest image by area in the album's folder, based on the premise that this is likely to be the best image.
In either case you could search and destroy the images in the folders after they are embedded if you don't want them.
tt2