How to change the banner in Movable Type’s default “Professional” theme.
Make a web optimized graphic using the same dimensions as the default chicken graphic.
(905 px wide by 170 px high) Then simply add “.old” to the original image, upload the new
graphic to your MT site, renaming it the same as the original.
There is also an excellent tutorial (a bit more in depth than my hack) on Tom McGee’s blog.