Use of SelectClipRgn

Hello.  I'm trying to superimpose one figure on top of the other.  The figure below will scroll underneath the figure on the bottom.  My approach is this:

One picturebox (A) that contains two pictureboxes (B and C).  I draw my background on picturebox B, which I move in accordance to scrollbars (w/o having to re-draw).  That works great.  I want one static, irregularly-shaped picture (in picturebox C) to sit on top of the scrolling picture, though.  This I am trying to do by applying SelectClipRgn to the picturebox C so that you can see picturebox B scrolling behind it.  

The problem is the clipped region behind picture does not expose Picturebox B, but the *windows desktop*.  It allows the user to see all the way through the app.  How can I tell VB to not re-draw picturebox C, but to redraw the scrolled picturebox B, in the clipped non-rectangular region?

Thanks very much.