I'm using the Kaboom Bros Graffiti layout by Mixit. I was able to change my header and get the height and width perfect with little to no problems. My only trouble is that I want to center the header image and move the navigation links and journal title all the way over to the left and up maybe an inch or less, so it's not covering the writing on my graphic. I have a Plus Account.
EDIT: Looks like the problem is solved for now. Thanks to everyone for their help!