Album art cover flaw

well, if you have all songs in library on itunes, and on your ipod, no matter what it is, you don't have any problem. but if you backup few albums, delete from itunes, but you have still them in ipod. next time when you try to resync album art because you would have little problem or improve navigation performance, deleted albums or songs in itunes will not add to ipod even they all existed. this is problem. no matter what itunes doesn't have songs, but ipod have. ipod should show album art. but it's not. manually I manage songs.