Right click on the Server Name at the top of TeamSpeak and select the option "Edit Virtual Server".
Get More Options
Click on the "More" button to display the advanced features inside of TeamSpeak 3.
Changing the Banner
To change the Host Banner you'll need to change the "Banner Gfx URL" to the URL of where you banner is hosted. You'll set the link of where the banner sends you under the "URL:" option.