I am a Mac user so things do get a little more complicated but here goes, my current solution is as follows (sounds complicated but is very easy in practice):
Firstly, I use XLD to rip my CDs into FLAC format. This works fine for me and produces, in a good few cases, superior quality to the CD played on my Uniti (error correction etc I suspect). I have a few AIFF files on iTunes which I like to have on my NAS and I use Switch for this which is also a very handy audio tag editor if your ripped music doesnt display as you'd like on n-Stream. I am also using Majestic to play my iTunes folder directly but this is really only for background music.
I added a folder called 'Naim' (you can call it what you like) in the pre-existing 'Media' folder on ReadyNAS. Inside that i create, as I go along, a simple folder structure of Artist or Composer names and inside those folders Album name folders. So, looking at the NAS on the computer the directory tree would look something like - ReadyNAS > Media > Naim > Bob Dylan > Modern Times
I pop jpeg artwork files into the album folders and name them 'Cover' and (through trial and error) follow the following procedure:
Log into the ReadyNAS and navigate to Services > Streaming Services, go to ReadyDNLA and select 'Rescan media files' and wait until it reports that the scan is complete. Then go to tools in n-Stream and hit clear image cache.
All the album artwork will now appear where appropriate. I am impatient and tend to scroll through everything and, as I go along, the artwork is added to the cache. I am not sure whether, if I waited, this would happen automatically.
A further refinement is to add jpeg artwork files to the Artist/ composer folders and name them 'Cover' too. This doesn't have to be album art, it can be anything you choose as long as it is named 'Cover'. This will then show when you browse 'All Music' or 'Folders' in n-Stream.
There may be a more e,egant way of doing this but it works for me and is easy and elegant to use. If anyone has a better idea I'd be very interested in knowing!