The following tutorial covers changing of the logo on a Plone 4.2 website with GreenJet theme installed.
GreenJet diazo Plone Theme uses default Plone 4 logo, displayed at the left top left area. Its width is 215px, and hight - 56px.
You can replace this logo with yours via Zope Management Interface. Open portal_skins -> sunburst_images folder, there is logo.png image. To set your own logo click Customize button.
Then browse for a new logo image you want to upload from your computer and click Upload button. Now you see your new logo.png image, and it is automatically put into custom folder, which means that for the subsequent logo replacement you should go directly to custom folder and upload new logo image there.
Now when you return to Plone interface, you'll see new logo image.
If your new logo image is higher than default one - whole top area will be expanded.