iTunes has some quirks. One is that if you add artwork using its Get Album Artwork function, the images are not embedded in the music files but are kept elsewhere in the iTunes database. This is normally fine. But if you want to send someone an mp3 you have, the artwork won’t travel with it. So I was looking for software to fix this and came upon the resource of Doug’s AppleScripts for iTunes (for Mac only).
Doug has software and scripts to help you with many common issues such as dealing with duplicate files and multiple iTunes libraries.
My problem was solved with the script Embed Artwork. Once you install the files, you get a script option in the iTunes menu as shown here, allowing you to embed the artwork into the MP3 file so that it is portable. This also helps people who want to use iTunes and other non-Apple media players such as Android devices.