I am not to hip on creating my own theme to I picked one and simply replaced the jpg banner at the top with my custom jpg, giving it the same name placing it in the same directory. I do not know if it is the same size. It shows up in Safari, but not in IE on Mac. So it probably is not showing up in Windows IE either. It is just showing white nothingness.
The theme I chose also had graphic buttons at the top, and they did not show up either in IE, only in Safari. The text links that appear below the buttons do appear. These where in the header file. The only thing I changed with these is where they link to, and the titles.