There are two ways on putting your banner.
a.
1.First go to Admin>Current Theme
2.Then you can see a text beside a text box with this text : Logo Image Url:
3.Then paste the direct URL of the link of your banner and click Save.
4.Then, Voila! After you save it, It will appear on the main title of your forum.
b.
1.Go to Admin>Manage Styles>Edit Headers/Footers.
2.Copy-paste the HTML code of your forums banner.
*If you want your forum banner to be centered, use the code below.
<center>CODE OF BANNER</center>
3.Click save then after the page load, You will see the forum banner at the very top of your forum.
I hope I helped you.