Easytag only support the old way, through the COVERART tag. but it's deprecated. the official way is through METADATA_BLOCK_PICTURE.
See: http://wiki.xiph.org/index.php/VorbisComment#Cover_art
i don't know where to report bugs for easytag, so i report it here.
xmms2 also have this problem, but it almost solved.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=565479
http://bugs.xmms2.xmms.se/view.php?id=2398
easytag bug reported in sf:
https://sourceforge.net/tracker/?func=detail&aid=2837960&group_id=5216&atid=105216
AFAIK, exaile and kid already solved this problem.