In TS3, you have the option of adding an image to a channel.
This is done using a BBCode.

To do so, simply enter the following code in the description of the channel:

[img]http://www.URL-of-the-image.com/Image.jpg[/img]

Please note that you must ALWAYS link to the image. http://www.host.com/image.php?ID=abcdef will not work!

You can find more information about BBCode at https://www.phpbb.com/community/faq.php?mode=bbcode