There seem to be a few posts around the 'net about album art issues on Synology NAS. One from this forum a few months ago suggests:
To get this to work properly for me I had to do the following:
1. Close all apps on ipad/iphone
2. Launch nstream on the ipad/iphone device and connect to media server
3. Now goto diskstation manager and in Media Server device list hit the reset button to clear everything and then refresh. It should find both the Naim streamer and the ipad/iphone.
4. Change the profile on both to "Naim n-Stream" and select 'Save'
5. Go back to the iPad/iPhone and clear both the upnp and image caches.
6. Relaunch nstream and the new image cache should be built with the original cover and the settings should stick
I found that if there were other apps on the iPad at first it didn't seem to work properly (I guess it wasn't seeing it as a 'n-stream' device).