Open Photoshop and open the two (preferably same sized) images to be animated.

Drag one image on other to get the following layer structure.

File > Jump To > Image Ready 3.0
Make Frames from Layers.
In the animation palette select 1st frame and tween it with the next frame for opacity.
Next, select the last frame and tween it with the first frame for opacity.

Set the timings for frames.
Set the no. of time you want to play the animation.
Check the performance in the browser
File > Preview In > Click the browser you want

Save the file with File > Save Optimized or File > Save Optimized As