I used the Import Images as Planes addon to import an image. It works fine but how do I change the transparency? I want to animate the plane so that it fades in and out. Here is my current node setup:
You just need to add another Mix Shader node mixing the output of the first Mix Shader node with another transparency shader. Then you can keyframe the Fac amount on the new Mix Shader node to fade out the object (or more precisely to fade out this material wherever it is applied to the object).
As an extra note, you may want to control the speed of the fade by editing the Fac value transition in the Graph editor.