At this point, save the file, then delete the GuideLayer, we don't need it anymore.  If you have done everything correctly at this point, the AnimLayer should look like below:

Make a new layer, name it AnimLayerMask.  
Duplicate the content of AnimLayer into AnimLayerMask.
Arrange the layer so that AnimLayerMask is located above AnimLayer.

Make yet another layer, name it: ImageLayer.
Arrange the layers so that ImageLayer is in the middle of AnimLayerMask and AnimLayer.

While ImageLayer is active, import a jpg file, then position and resize it so that it fits within the paper. (Hide AnimLayerMask so you can see the image.)

Hide all layers except AnimLayerMask
Go to Frame 1 of AnimMaskLayer and delete color-fill on the non-gray colored area, also delete the diagonal line on the bottom-right (see below). 


Go to Frame 20 of AnimMaskLayer and delete the color-fill on the non gray-colored area (see below).  

On Key frame 21 and 40, delete everything, except the outlines.

The purpose of this exercise is to create a mask so that the image on front side of the paper will show through the animation.  

Click on AnimLayerMask, then make it a Mask layer.

If you go to frame 10, this is how it should look like.

What is going on?
Here's a little trick with mask.  

Test the movie.

You can easily reveal another picture behind the paper.  See here.



(C) F. Permadi