I'm new to messing with vB, but what you are looking for is in the "header" template between:
<!-- logo and buttons -->
<!-- toplinks -->
which is at the very top of the template. I removed everything between those and just built a full screen gif into a table (used Dreamweaver). Make sure you store your *.gif/jpg in the forums/images folder and use variable { imagesfolder }/gif/jpg name. (minus the space).
As I said, I'm new, so wait on more replies before you try it and as always, back up your database before hacking