Is this possible please.
I want to have a forum that opens with a header image I will create which runs right across the top of the page.
Then i want to have 3 boards all with different custom, full size headers which will give each its own look and identity.
I think what you are talking about can be done with the template system.
This is how I would do it.
Make three copies of your theme showing the different banners.
Then go to admin.
Manage Boards
Select 1st board.
then look for the Board theme option and select the theme you built with the banner for the 1st board.
Then do the same thing for the other two.
Hope that helps you... if you need anything else ask :)