I had the same problem. My solution was to create a new text box with the following code in it:
<img src="PATH_TO_BANNER_FILE" width="BANNER_WIDTH" height="BANNER_HEIGHT" alt="Madcap Heiress" />
Replace PATH_TO_BANNER_FILE with the URL path to the banner file
Replace BANNER_WIDTH with a number representing the width of the banner (in pixels)
Replace BANNER_HEIGHT with a number representing the height of the banner (in pixels)
Then SHIFT+drag the new box onto each of your theme template (Home, Single, etc...)
I hope this makes sense and helps.
You can have a look at my website - which has the same theme as yours - to see the result: