Hi Kevin,
if I was to do it I would first try to use the MoGraph Multi-Shader in Combination with a Random Effector on a Cloner object to achieve your goal.
First make a Cloner object to create your Post Cards, then connect a Random Effector to this cloner object and disable Position changes. Turn on Color Mode and set it to add. Then create a new material and select MoGraph->Multi-Shader in the drop down menu of Color. Click on it and for shader 1 choose the first picture of your library with the postcards. Then click on the picture and choose calculate. The Multi-Shader now loads all the pictures in the directory because he thinks it’s a sequence. Now click the arrow up to get into the Multi-Shader properties again and hit “to layers” he will insert all the other pictures of the library and attach the material to your cloner object.
Hope this helps