A network streamer (or any standard UPnP client) does not interface directly with iTunes. The device is a UPnP client and as such, it plays back music from a computer or NAS drive. This is where the confusion lies because of course your iTunes music library will be stored on your computer or NAS drive too.
So, the network streamer will play back the music files from your 'Music' folder. This same 'Music' folder may also be your iTunes library, but there is no direct link or connection between iTunes and the network streamer. Also, any playlists or album art that are created in iTunes will not be 'sent' to the library, as iTunes itself creates these and stores them in its own directories.
In order to access album art and other track information (tags), this must be stored in the music file's own metadata - iTunes does not do this for you. There are many tagging programmes available (for example Media Monkey or dbpoweramp) which can do this. See FAQ here
If you are using a NAS drive to store your iTunes media library (rather than on your computer's own hard drive) an alternative may be to use a NAS drive which supports an 'iTunes server' feature. This is an application built into a NAS drive that 'shares' your iTunes library including playlists, etc. This is dependent on the individual NAS drive and exact features may vary.