To creante an image placeholder, you need to set the alternative text of the image. You can read how to do this on the practical page:
In order to do this, just right click on it, open the image properties and search for the field alternate text (this name may vary in each version of Word MS). Type in this field the name of the placeholder, for example: $IMAGE$.
and the API page:
We also recommend you to check the included sample in the package that illustrates how to use the method.