Recommend Looking Good in Print by Roger C Parker to anyone arranging text and graphics. It relates to human interface design and ergonomics but with a focus on legible and attractive presentation of visual data.
Thank you for your feedback! I've also thought about this and would also like to see a cover window with the same width as the Tag Panel.
I'm just not sure if this is a change that is welcomed by most Mp3tag users, mainly because some are optimizing vertical space or are using a very wide Tag Panel (which would make the cover really big in some cases).
In addition, I'd put cover art metadata and navigation below the cover art. Covers can have descriptions and displaying those might exceed the available space above. Also, navigation is only visible for files with multiple covers, so showing/hiding it would make the UI jump or just leave blank space. This is better be done below.