I'm trying to show a specific icon when my qtoolbutton is checked - so far without success.

My workflow:
1. create a qmaindindow in qdesigner
2. add a toolbar
3. add an action and make it checkable
4. add an icon to the action for all states (normal off/on, active off/on, selected off/on). All icons are in a resource file.
5. test it.

At first the default (normal) icon is shown. On mouseover the active icon is shown. So far so good. But when I click my qtoolbutton and it is in its checked state the active icon is shown again. Why is the active icon shown and not the selected icon? How can I change this behaviour?