Yeah, you're right, you would have to create your own image.
There is a free software called GIMP, which is pretty decent:
http://gimp.org/
When you create the image, though, make sure that the background of the image matches the background, so it wouldn't stand out. From the source code, it looks like the background color is #90A1B9
Hope that helps.
Sherif
EDIT: just to clarify, when I say background color, of course, I mean the background color for the header, which is different that the background color for the content area. I hope this makes sense.