How do I change the header image in Plone?

YouEnter should be inthe the Zope interface.interface by adding /manage to the end of your URL.
Go to the folder ‘/portal_skins/plone_images/logo.jpg. Customize,portal_skins’ and the subfolder ‘/plone_images’. Click on ‘logo.jpg’ Click on Customize and you will be directed to the /custom folder where you can upload your headerlogo image.

TaFor da!more info: http://plone.org/documentation/how-to/custom-logo