Hi - I searched but could not find help that worked on this.
I have all my album art stored in the album folder as folder.jpg on a synology NAS (previously Netgear - no difference).
At the album level (eg looking at tracks in the specific album) in the app it works fine, but when I browse at the level up (eg all the albums by a specific artist) there is no art for each album.
Per other posts - I have activated media server - browsing settings - display cover image in settings for the NAS.
I dont know if this is the answer, but can I suggest the uPnP server software called Asset which is so much better for HiFi Audio Streaming tan the base product. You will get a 1 month free trial, so easy to test. There is a Synology version, with instruction.
Artwork shows for me using the Synology server as long as you are not looking at the folder view. If you browse metadata you should see the artwork which is part of that metadata. In other words, look under ‘Artist/Album’, not ‘By Folder’
I used Synology’s ‘Media Server’ for a while.
I don’t know if it’s changed since, but in my days it only had Folder View and very limited art display functions …. amongst a few other limitations.
I tried Media Server and when Illustrate released an Asset for Synology, then I tried Asset UPnP.
Both were a country mile better than Syn Media Server.
My final choice was Asset, it does it all and more than I need.
Running a good UPnP server such as Asset or MinimServer is the answer. I have both on my Synology NAS and would recommend Minim if the OP has a significant collection of classical music. But it does have a something of a learning curve to get it to perform at its best so for other genres Asset might be better.
For me, if I could only keep one, it would be Minim.