I found that I for customizing the quick element toolbar I need to edit the file quick_element.xml in the folder C:\Users\<user name>\AppData\Roaming\Adobe\FrameMaker\12\WorkSpaces\Structured\WYSIWYGView\toolbars. The location given in the online help (http://help.adobe.com/en_US/framemaker/using/WS2d2a17056e219198-10de064142a20156dc-7fff.ht ml) does not work.
It turns out now that my custom icons only appear when I edit an existing XML file. When I create a new one using the DITA menu, they are not displayed. As soon as I save the new file and open it, the icons appear again.. is this a bug?
Robert