1. Open animation panel (Window>Animation)
2. have the 2 images on a single layer (image>apply image on a new layer for each)
3. make 1 visible and hide the other one
4. make a new frame and do the opposite of number 3
5. "Tween animation frames" and select a number you desire (5-30)
Outcome:
(it is slow because it has more than 67 frames... only 20-30 is needed
re-did it and halved its file size (22 frames now)... as you can see it is faster now
)
P.S. if the above is not understood, PM me and i will make a longer tut with images to guide you