Make your banner the size of the width of the forum.

Upload the banner to the location:

sitename.com/<rootpath>/images/misc/<filename.ext>   (no angel brackets)

Go to the admincp

Admin Control Panel > Styles & Templates > Style Manager > dropdown: StyleVars > Image Paths > Title Image

And change the name of the file there to match your file name – including changing the extension.

Save and reload your website.

If you have any questions – feel free to contact me. For an example please take a look at http://fe9.org

Admin@fayax.com