How to put a 728×90 ad above header image?

I am trying to place a 728×90 ad block above my blogs main header image. I’ve seen this done on other blogs, so I know that it can be done. Unfortunately, I have no idea how to do it.

I am using the Easy Adsense widget, but it only has options for Adsense units within a post or on the sidebars. There is no option for placing a unit above the header. So I assume that I’m going to have to do it in the editor or CSS style sheet.

I’m using the Prosumer theme. My blog can be found here:

Can someone please help me on this one? Thanks.