If you exit the app, Mp3tag remembers the state.
So, if you enable the Tag Panel (Ctrl + Q or Menu: View > Tag Panel) and then exit Mp3tag, the panel - including the cover art section - will reappear automatically when you next start Mp3tag.
The same is true for the position of the Tag Panel.
Please take a look at this guide on how to manually move the tag panel to its 4 useful positions: