Are you saying that the artwork you've downloaded from Qobuz isn't displaying through 'Home Music Share' in the WiiM app but the artwork from TIDAL is, even though they're both called cover.jpg?
I'd test it in another UPnP control point first (BubbleUPnP, Hi-Fi Cast, mconnect) just to make sure it's being sent correctly from the QNAP media server, and if that works, I guess I'd try and identify the difference between the two sets of files (mediainfo may be able to help), maybe one is a different format e.g. JPEG 2000, although I'd be surprised.