Adding to Jumpy's reply, one technique that is used with pictures takes advantage of the z-order. That is, the front picture will either hide or show the picture behind it. Thus, you only need to control the visibility of the front picture. But, it could just as easily be applied to both shapes. It is based upon a technique from David Parker. The hide / show action of the menu sets a parameter, could be in the user section of the shapesheet, to either "1" or "0". This value is pushed into the shapesheet of the front picture, in the Foreign Image Info section and is used as a multiplier in the image width and length cells. You should (could) see a formula in those cells, something like Width*1*Sheet.1!User.ShowFront. Sheet.1 is the top level, group shape. Thus, the hide action would set the user.showfront to zero, and would collapse the front shape, showing the back shape.
HTH
Wapperdude