Hello,
once you uploaded your image files and add the XML file you can look in your ACP and find the styles section..
once you click the style you want to edit you can see the templates like the "header" or the "footer".
These are all edited by html, I suggest making your board have two styles. One that is the main one ppl will see, and one to play with. Then if you edit the one your playing with, and it's how you like, you can just copy the template to the live style..
The best way to figure things out is to just play with it.

I'm heading to class now, or I'd add some screen shots to help explain things.. If needed, I can add those later.
-Brandon