I'm about to test this out.
Just finished adding 500x500 cover.jpg album art to every album i have on the X5. Most in flac, one in mp3, several in m4a/aac. I'll update this post in a bit.
You have said that you have embedded art with .ogg when this isnt supported.
Embedded art takes precedence over cover.jpg - so does cover.jpg work with .ogg in 2.2 that doesn't have embedded art?
Maybe the prsence of the embedded art is blocking cover.jpg as it would with .flac files.