The "Unknown" bit means bliss couldn't assess the album's compliance. This is because it failed to read the existing art to check its dimensions.
I'm working on this. The cause are JPEG album art images that cannot be handled by the imaging software that bliss uses (the default Java imaging API, for the technically curious). For now, the only workaround is to change the errant album art yourself manually in a music tagger.
Any other FAQs I forgot or clarifications required? Post your ideas below!