Compositing Bitmaps

I have rendered an RGB bitmap to a quad.
A TIFF RGBA image has been loaded and the alpha value of black pixels set to zero.(the TIFF loader insisted on setting all pixel alpha’s to ‘1’).
It is the same size as the previously rendered image.
I am now trying to render all the non-black (i.e.opaque) pixels over the existing image.
All attempts using blending, alpha tests and setting glTexEnv have got nowhere. Very little info on the Net.

Can anyone help ?


   David Sykes