The following tutorial covers replacing the theme logo in Twirl diazo theme on a Plone 4.2 website.
Twirl theme comes with default Plone logo. It is a transparent image that is displayed above logo background image. You can replace both logo image and logo background image.
You can replace this logo with yours via ZMI. Open portal_skins -> sunburst_images folder, there is logo.png image. To set your own logo press Customize. Now click Browse, select the logo image you want to upload from your computer and click Upload:
New logo.png image was automatically put into portal_skins/custom folder, which means that for the subsequent logo replacement you should directly go to custom and upload new logo image there.
