The 2.0 fimware for the Cybook and Opus have new thumbnails for epub files. They use the .thn extension and it is append after the .epub extension. This is unlike the _6090.t2b thumbnails which use the book name without the extension and _6090.t2b appended to it. I have yet to start figuring out this new format but at first glance it looks to be similar to the _6090.t2b files.
What I have found is, if a _6090.t2b file is present that will be used and the .thn file will not be generated and the _6090.t2b will be used as the thumbnail. However, if both the _6090.t2b and .thn are present then the .thn will be used.