Editing track metadata or adding album art in iTunes only stores those changes within iTunes, making it easier to browse, etc if you're actually using iTunes as the interface.
However, iTunes doesn't save the changes to the actual file, so it doesn't help if you're not using iTunes (eg. using a UPnP server instead) to browse the library. So, you may think that you have edited your track metadata (tags) by using iTunes, but of course you haven't actually.
To edit your track metadata correctly, please see the FAQ here