I’m very tired so I could be wrong.. However, my suggestion would be:
With any editor of your choice open the file sidebar.php in the wp-content/themes/coppermine/ folder.
Insert your banner code after the <hr /> tag at about the end of the document.
Save and upload.
You have probably already tried this.. However.. I hope that does it for you ??
I’m actually inserting banners on my own blog right now. Perhaps you can get some inspiration from my source code: https://www.gudmann.net.