First, which theme are you using? If it is the Effervecence+ theme the answer is simple. First, update to the current nightly build. Then change the zen-logo.jpg image you will find in the efferevescence+ images folder.
If you are not using Effervescence+, also install the nightly build. Then look to see what image is now showing. It should be one of the images described here http://www.zenphoto.org/2008/05/theming-tutorial/#special-images. Follow the instructions there to replace the image that is showing.
NOTE: this is a new capability and therefore the nightly build is required to implement this replacement.