I use Twonky 4.4.2 with a Linux box and a PS3 (Firmware 2.0). To organize my MP3 libary I use iTunes. I found 2 problems with displaying album art:
1. The album art of MP3's with ID3 tags with version 2.2 wasn't displayed. After conversion to version 2.3 with iTunes, the album art was displayed. It seems, that coverart was displayed correctly, when the ID3 tags were of version 1.1, but I didn't check a lot of samples and converted all my files to version 2.3 anyway. I didn't try to use version 2.4, because I wanted to avoid the feature of that version to store the tags at the end of the file - while making edits to tags faster it wouldn't help programs like twonky to read the tags as fast as possible.
2. When I used copy/paste to copy the album art from the internet browser into the cover art field in iTunes, the art was not visible on my PS3. When I saved the image to disk and used iTunes to load it from there, it worked without any problems.
By converting to ID3v2.3 and following the workaround for inserting the art, I managed to display cover art regardless the size of the image (I tested 200x200 - 700x700 pixel).
None of these problems affected the display of cover art in iTunes in any way.
BTW, when working with iTunes you have to check, that the cover art and other information are stored within the MP3 file and not within the iTunes database (where Twonky cannot read them). When I accidentially removed the ID3 tags from some of my MP3's, iTunes transferred the contained information to its internal database. Luckily it transferred most of the information back into the file, when I added the ID3 tags again. The only exception was the cover art, which I had to copy manually to transfer it from the db back into the file.
Conclusion:
- Use ID3v2.3 tags
- At least with iTunes don't directly copy/paste coverart from the internet browser, but insert it from a file.
I hope my findings help the users to get the cover art displayed and give the developers a hint how to fix the bugs in future versions.