The following tutorial explains how you can change the top image on a Plone 4.2 website with Twirl theme installed.
Twirl theme allows to have different top images for different site sections. The recommended image dimensions are 370px width and 150px height.
To set your own top image, add Image with shortname/id topimage to the desirable location. Note, add an image to the site root and it will be displayed everywhere on site, if added to certain folder, then the image will be displayed in this section only. If your image size exceeds these default dimensions, it will have its corners automatically rounded to fit the space.
Now when you return to home page, you'll see new logo background image.
