Is this for local audio files I presume? How have you ripped them and what software did you use?
I use dBpoweramp and rip to FLAC, it adds album artwork as Folder.JPG in with audio files and it also embeds a copy into the FLAC file too. - I've never had an issue, used it on PC and now Mac.
They also make a bit of software called PerfectTunes that has an album artwork part to it, you would point at the NAS and see if the files are missing the artwork etc. Believe you would need to pay for it to save the artwork, but if you've lots of albums and tracks it would likely be worth it as it has an auto 1 click fix up or something like that. - Worth looking into even if you use it unregistered to just identify those albums with issues and fix up manually if it's not too many.
For what (little?) it's worth, I believe that only .jpg images of less than 500kb can be displayed.