help on using toolbar AddCheckTool

Hi,
I am trying to use a CheckTool in a toolbar I have built. I would like the image on the button to change depending on whether the tool is in the 'toggled' or 'untoggled' position. I tried

toolbar.AddCheckTool(toolId, bmp1, bmp2, short_help, long_help)

thinking that bmp1 would be used in one case and bmp2 would be used for the other. Unfortunately, bmp1 is displayed in both cases. Clearly, I'm missing something and am having another bout of cluelessness.

The wxPython demo (v2.5.1.5) has this particular case commented out for some reason and I haven't found anything useful by Googling or by looking in the wxWindows docs.

How do I manage to change images depending on the state of the checktool?

Thanks for your help.

-g