iTunes: How to manually add artwork

I've been organizing my music and find that when an album is a compilation (i.e. Soundtrack) I can't get the Album Artwork to download. The albums are found on iTunes though I didn't purchase them on iTunes but that doesn't seem to matter with other single artist/group albums. For example, I have the soundtrack to 'Elizabethtown'. It's found on iTunes and all my songs and artists match, but I can't for the life of me get the artwork! I've tried adding (Original Motion Picture Soundtrack or whatever else is in the title of the CD showing on the iTunes page.) I can't reimport the album as I no longer have the original CD - borrowed from a friend! (Did I just answer my own question...?)

I don't want to have to go in and change all the album names to the original albums the songs are found on unless I have to. (I've found that if I can't get the artwork for a particular album, I search iTunes for the same song, different album and it usually uses that artwork when I change the name of the album in my Library).

Is it impossible to get artwork sometimes? Or should I be doing something else while editing album names? Or is the editing what is preventing me from getting the correct album name, etc.?
THANKS!
Meagan