Interesting – where’s the art from the JRiver screenshot stored? Or is it cover.jpg?
If so, I’m guessing you have a larger image (the 882x768 one from the Roon screenshot) stored in that Artwork folder, and named something that makes Roon think it’s the front cover (like front.jpg or cover.jpg) – in that case Roon will pick the largest front cover art available.
Let me know if that sounds right. We do have some feature requests for more control over which cover art we use, so I would look these over and make sure you let us know what you’re looking for.