The easiest way to do this is to have 2 photos of the same size. You can either have them, or resize them. Make sure they are the same shape also. Once you have them ready. You will may want to put a frame on them. In this tutorial, my pictures are the same size, and I have added a gold frame to both pictures. Since it is the same frame, you wont notice the fade effect on it. You could probably use different frames, that is up to you. I used a frame plugin to make my frame, you could also make one by inserting a rectangle/square shape and cutting the inside out of it. Fill with color and bevel, but that is up to you. The frame plugin I used is free and you can download it HERE!
Once you have them frame, you need to click in the stack and drag the picture you don't see in your work area over to the one you do see.

Once you have done that. You will need to make sure you place the one picture on top of the other so you can not see the picture on the bottom.

At this point. You will save this as frame one for your animation.
Now to do the fading, you will use the Even Transparency tool.

I am moving the lever over to 20 each time. Now this is up to you. Depending how many frames you want in your animation. Remember for every frame you make you will need to double it, so that it is a smooth fade.
Here I have moved it to 20, and you can just start to see the picture under it.

Now you repeat the transparency, and save as the next frame, until you see only the bottom picture. Like I said you can shorten the amount of frames in your animation by maybe moving the transparency lever to 40 a couple of times. In mine, I have applied 20 to the first 5, then at 6 & 7 I did 40, then at frame 8, I just right clicked and deleted the top picture, so all you have is the bottom picture. At this point, you want to actually work with your undo button.

Because you want to fade it back to your first top picture. So you will now click undo once, and save. Click undo again and save, you repeat this all the way back to the first picture. My animation will have 15 frames. Yours may have more or less, depending on how much you moved your lever. Once you can your frames you will animate them in flipbook animation. Close out everything you have open in your program.
Now you want to open all your frames.

And now go to flipbook.

You will click through next twice, and come to your size. If you want your animation the size it was you will have to change it here. Just click the drop down menu, it should be there.

Or you could choose any of the other sizes here. When you are happy with the size, click next. This is where you choose how fast your animation will move. The default is 3.0.

If you make the number lower than 3.0 it gets faster, higher than 3.0, it gets slower. You can acutally click the number, and then click next, if you like it save it, if not, then just click back and change the number and try again. Once you are happy, save it! TaDa!!