I get my album art from different sources iTMS, Amazon.com, Amazon.co.uk, Amazon.de,
Google Images and
Discdogs are the most usual sources, but I've actually have had to use a scanner every now and then.
Album art fetched from iTMS for imported music is stored within ~/Music/iTunes/Album Artwork/.
Artwork pasted into the library from other places
(or technically from iTMS if you do a cut and paste with the downloaded art) is embedded into the music files, and as long as you've backed up all your music
(and preferably the Music Library file, even though it doesn't actually have anything to do with album art) it will still be there after a reinstall.
(On a related note: Reinstalling Mac OS X is very rarely needed! If you have specific problems, try to get help for those. And even if you have to reinstall, in most cases you can use the Archive and Install option that reinstalls the system without touching your data (including your music) or 3rd party applications. Still: Backup is a good idea, even then...

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