The header image by default is a subtle image of a «floor plan». There is no way to change this in the Theme Options currently, but it’s not difficult to change. The image is called header.png and it’s stored in the /images folder. It’s 960×152 in size. Feel free to create any header image you want that is the same size. Save it in the /images folder with the name header.png. If you name it something else, then you must edit the style.css. (In style.css, change the file name for the «#header» selector. Should be self explanatory when you see it.)