* run updater/updatedatabase to create imageurl field in itemtypes.
* go to Koha >> parameters >> itemtypes >> modify (or add) an itemtype. You will see around 20 nice images to choose between (thanks to owen). If you prefer your own image, you also can type a complete url (http://www.myserver.lib/path/to/my/image.gif)
* go to OPAC, and search something. In the result list, you now have the picture instead of the text itemtype.