Ogg Vorbis embedded artwork

[simbun]

I've been playing about the last couple of days trying to transition away from MP3, and had largely settled on Ogg Vorbis (as Opus is way too modern for the music industry it turns out). My source files are FLAC, but I make copies for portable devices and also move some to Sonos, mainly for the girlfriend.
I had been testing with Sonos and a Fiio portable player, and everything was compatible (in the sense that Sonos was as bad at dealing with Ogg Vorbis as it was with anything else), but then I finally tried with MinimServer and I don't see any album art.
I converted these tracks with foobar, and they're the same images that work fine with MinimServer in FLAC files so it's not an image format problem. I also did a FLAC conversion with "fre:ac" and it has the same problem.

Is this a known problem?
I guess it's not really important as I don't think I'll ever have any served by MinimServer, I was just shocked :-)

EDIT: Thinking about it, this is probably a control point issue as I doubt the artwork is sent to the control point seperately! I'm using BubbleUPnP so I'll try there.