Just right click on the image that's there now, and then follow the menus to the last item in both. Create your design and save as a .png. I've found that around 500px with transparent background and white text works nice, but it's all up to you of course. Add that file to the Panel End Marks Folder, restart Lightroom, and it should appear. Magic! This along with the Identity Plate can be a great way to keep your branding consistent if you use Lightroom to show work to your clients.