Publisher’s diazo theme comes with default Plone logo.
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 button.
Now press Browse button to select the logo image you want to upload from your computer and click on Upload.
New logo.png image will be 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.
Now when you return to the Plone website, you'll see new logo image.