Read the following tutorial and learn how you can change a logo in Sliced Plone theme.
Sliced 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.
There are situations when logotypes in the upper right corner and on the top image have to differ due to specific background colors or images. In this case you can use one logo for the upper right corner (via customizations described above) and other logo for the top image. For the latter add image to the root folder with ID logo_big.png. If you use dynamic navigation tabs it will be better to hide this image from navigation menu. For this go to the Edit tab of the image, click on the Settings tab and tick "Exclude from navigation" box.