how do I add a small banner to forum index? (not logo banner)

[arcadian_girl]

Hi I'd like to add a small, clickable banner to the area indicated in the image by the red box and red arrow. I've taken a look but at the templates but it's all Greek to me, I can't see where I'd add the code. Any help would be much appreciated

[Michael Biddle]

That would be in the navbar template