[development] tinymce and img_assist (icon problem)

I've followed the instructions for tinymce and img_assist, both of which seem to be installed properly. However, when I select the DrupalImage button, the related img_assist icon does not appear among the other icons. I can see the tinymce image icon, and even turn it off, but I cannot make the drupalimage / img_assist icon work. Any thoughts? Sean