On http://jfearn.fedorapeople.org/en-US/Publican/2.0/html/Users_Guide/sect-Users_Guide-Website.html in chapter 6.2, point 7 it is stated that "To customize the logo at the top of the navigation menu that provides a link back to the home page, create a PNG image 300 px × 100 px and name it web_home.png." The image should actually be called "web_logo.png" and be 290x100 pixels otherwise it doesn't work.
Oups, in my hurry I forgot to note that I'm using publican-2.1-0.fc12.noarch.
Thanks for the catch, Steven -- this will be fixed in the guide we ship with Publican 2.2