There is a bug (I think) in the vorbis library.
If the comment header is spanning multiple OGG pages, the libray fails to read all the comments.
I suppose it is not common, but I use JRiver MediaCenter which stores the album picture in the header of the OGG. While I know this not recommended by the spec, I don't think it is forbidden, either. Furthermore, I don't think there is something in the spec preventing the headers to span multiple page.
I have corrected the bug (or added the feature, whatever...) in the source and uploaded the files in the "Patches" section.