Rendering child control problem

Hello all,

I have a problem with my custom webcontrol. It contains an ImageMap child control, which doesn't render as needed.

Shortly, the function of the webcontrol is to expand some hierarchy when clicking on its nodes. The resulting hierarchy is drawn as a bitmap, generated in the RenderContents method of the webcontrol. Then it should be displayed by the ImageMap. ImageUrl property of the ImageMap is set to an ASPX page, which takes the picture from session variables.

On page load it works fine and the initial picture is displayed correctly. However, when I click on a picture to expand the hierarchy, it doesn't redraw. I've tracked it down and found that the RenderContents of the parent object is called and the new picture is generated. However, there is no call to the ASPX page, which passes the new picture to the ImageMap. It makes me think that the ImageMap simply doesn't know that it has to reload.

Is there a method to "invalidate" the ImageMap control so that it reloads the picture when the parent webcontrol is rendered?