iTunes uses a serious of files and folders to create the library you see. These are normally all contained within the central iTunes folder, though sometimes people split media off onto a separate drive.
For artwork iTunes places a separate file in the artwork folder and gives it a cryptic reference. When you click on a track iTunes looks up the track information in its .itl file. This tells iTunes what track information you have, where the file is located iTunes should play, what artwork file it should display, etc. If you used iTunes to fetch the artwork the only place the artwork is located is in this artwork folder. If you add artwork yourself a copy if placed in this folder and another copy is also embedded in the media file itself.
The interleaving of the various files and folders means it is best to leave it all up to iTunes and keep the lot in the iTunes folder as a complete package. If you do not and things are not done correctly, either by you messing up or the NAS or the operating system, it is possible to corrupt the library references. Using a NAS throws a wildcard into the game because for some reason NAS and iTunes do not play well together. Nobody who has had a problem with NAS and iTunes has actually gone to the trouble of systematically analyzing the issue so I can't tell you why. My guess is NAS tend to use a different file system and you have to trust it can correctly interpret file paths. It may be an issue of accents and you trying to speak to somebody who uses pidgin English..
The cat was a medium hair tortoiseshell who lived to be about 19.
