Without Formatting...
To add a banner to a chat room, choose Set Banner from the chat window menu, enter banner text and click OK:

With Formatting and Graphics...
Chat window banners can consist of any number of cells of any type, including the outputs of evaluations. Evaluate inputs in a “Set Banner...” dialog to produce banner cells, then delete inputs that are not part of the banner.
To create an output and add it as a banner to a chat room, choose Set Banner from the chat window menu, create the banner content and click OK:

- A banner may consist of more than one cell.
- Banner cells can be copied from other windows and pasted into the “Set Banner...” dialog.
Notes
Choose Remove Banner from the chat window menu to remove a banner.