Save your foreground figure render in psd or tiff format. Open your background render in Photoshop. Open your foreground render in Photoshop and open the Channels palette. Ctrl-click on the last channel (the alpha channel). Hold down the Shift key and drag the selection onto your background render. (the Shift key ensures the move will end up in the exact center of the target pic if both pics are the same size.) Wa-la. Note: I think the higher dpi you render at, the finer selection you get.