Please read http://wiki.xiph.org/index.php/VorbisComment#Cover_art for the official cover art comment format. In short, it is a base64 encoded version of the FLAC METADATA_BLOCK_PICTURE block, not the COVERART=… and COVERARTMIME=… comment that Jaikoz uses.
Ogg Vorbis cover art comment not to spec
This standard has was only introduced a few months ago, before that there was no standard except fot the unofficial one that Jaikoz and a few other applications use. I will move over to using the official spec in the future.