Would this be outside your banner area or are you talking about adding an image in the banner area?
If the banner area just go to: Admin> Themes> Advanced Styles & CSS> Visual Editor> and click Header(top), then Forum Banner and Banner Image. There you can upload your image/banner and set it how you want it to display.
I'm assuming you want to change the text in your banner area. To do that, on the same page you added the banner image you can change your logo text by clicking banner text. You will see all the options to changing the style of your text.
If you simply want an additional image to show on all theme you can go directly into your global or main header and paste the image link:
<center><img src="http://myimage.jpg" /></center>
Just in case I confused you a little the banner area is actually two parts. Like here on ProBoards you see the text Proboards Support. That is the logo area which can be text or an image. The blue background is where most add their banner and can be just a color or image.
Last Edit: Apr 21, 2014 15:54:53 GMT -8 by Tumbleweed