See if this thread sheds any light on the subject (scroll down towards the bottom to get to the issue):
http://yabb.jriver.com/interact/index.php?topic=98810.0I just bought the app, set up my server with static IP, forwarded the port and was thrilled until I tried to play a song. Now I am pissed off.

If this really is the case, then it should NOT still be a problem over a year later! The app should have a
big bold disclaimer on it that states it may not work unless you spend a lot of time and effort hunting down and resizing various bits of cover art. WTH??
UPDATE:

...and now I am THRILLED again. In my case, I changed the setting to use Transcode Audio 128k and it worked!

I was really dreading having to resize images (just the process of opening/saving as, blah blah blah) so I decided to randomly start changing settings to see what would happen and viola! FWIW, this thread is one of several in the "same vein" that came up when I searched for the error message and that was how I was assuming it had to do with cover art resolution.