Replace Gray Speckled Header Colour with Image

Hi there, I'm new to  using Simple Catch and am having some difficulty with the header. I would like to replace the gray speckled area with an image so that it aligns on the both sides with my background image and touches the pages bar (About, Disclosure, etc.). I have created a header image I would like to use, I just can't seem to make it fill in the whole area. My website is http://www.monkandmarvel.ca/ Any help you could offer would be greatly appreciated.