Flipping a photograph to an illustration

I want to to flip a photograph say of a bird to an illustration of the same bird, a bit like a rollover on a webpage, the only way I can think of doing it is to create two slides the same,one would have  the photograph and the other would have the illustration, and use the photograph as a kind of button that goes to the next slide, so it still has all the same content except that it will be the illustration.I have about 12 pictures, so a total of 24 slides, this seems like an awful lot of work, does anyone know of an interaction or an easier way of doing this?

many thanks

sue