.itc - Itunes CoverFlow artwork
iTunes keeps a cache for all the album artwork in its Album Artwork folder. The artwork is wrapped inside of .itc files and you can extract it with File Juicer. So far I have only seen JPG and PNG files in the artwork files, so to do a fast extraction it is sufficient to check the PNG and JPG checkboxes in File Juicers preferences.
File Juicer can also extract the cover art from mp3 files. It does not care if they are in ID3 tags or not - it will find images inside any file type as long as the images are in one of the common fomats File Juicer can recognize (JPEG, GIF, PNG, TIFF - see file Juicer's preferences).