How do I change the header image in Plone?

Enter the the Zope interface by adding /manage to the end of your URL.
Go to the folder ‘/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 logo image.

For more info: http://plone.org/documentation/how-to/custom-logo

If you have already changed the logo image and would like to change it to another, start in the folder /portal_skins/custom, click on logo.jpg and upload the new image.