Trouble Getting PageId form ImageSelector dialog

I made MediaContentSelectorView.aspx use my external template so that I could
do some filtering of the image libraries based on current page.

Trouble is... from the OnPageLoad inside the popup dialog control, I can't seem to get the pageid of the page that popped it up.

For a control directly on the page, I use  SiteMapNode currentNode = SiteMap.CurrentNode;  and get the ID from there.  In the dialog, currentNode returns  NULL.

How do I get the CurrentPageId from within the Image selector dialog ?

thanks,
Tim