The idea of the *Compositor* is editing some kind of **Input**. Either the **rendered result** via *Render Layer Node* or an arbitrary **image** from your hard drive via *Image Node*. 

At the moment it's only possible to set up an **empty plate** in the compositor via *Mask Node*. 


[![enter image description here][2]][2]
*Automatic animation via Time Node*


**Note:** It's not necessary to use a *Time Node* (in your case), because it's also possible to animate the `Fac` value of the *Mix Node*, related: http://blender.stackexchange.com/questions/34660/is-it-possible-animate-the-compositor-node-parameters/34665#34665 

[![enter image description here][1]][1]
*`Fac` value animation*


  [1]: https://i.sstatic.net/yLwlF.png
  [2]: https://i.sstatic.net/JpeH0.png