I had a similar problem, where I wanted a background image in the header behind the header text, above the menu. I figured out the following code in css: (I use the twenty-twelve template)
image.url is the url to the image you want to use.
You can also add
if you don’t want the image to repeat it-self.
I hope this helps.
You don’t need the “box-shadow” rule from that example, and you should probably change the padding-top rule to something like “margin-top:260px;” instead. Try it out. Note that you will need to adjust the home page separately using the “.home” selector since your home page header image is a different size.