You could place it in several locations.
1) In the actual ubbt_header.tmpl file right after this...
<? //UBBTREMARK
2) Paste the code somewhere in ubbt.inc.php
3) Save it as an include like you mentioned, and then paste this into ubbt.inc.php...
include ("$thispath/includes/banner.inc.php");
...right underneath the others listed...
include("$configdir/config.inc.php");
include("$thispath/mysql.inc.php");
include("$thispath/theme.inc.php");
I would personally use the 3rd method just to keep everything orderly, and allow easier editing of the html code.
