First of all - what do you mean with masthead? The German "Impressum"? If yes, an image is not really the best choise (the Telemediengesetz says that it has to be barrier-free AFAIK). Or do you mean something like a general headline you want to display on all pages?
Then: if you want to edit the layout, you have to edit the template you are using. It is located below /lib/tpl/[yourtmpl]/. The different templates may use different ways to display the content. I never used the
usable template, but it seems to be not maintained anymore (Last update: 2009-04-13). Many new features like the Media Manager won't work with it. However, you should place yout edits in lib/tpl/usable/main.php.
Can you provide a link? I'm sure there is somebody who can help you out if we can look at the wiki itself. :-)