Unfortunately, DandelO, this issue is still not resolved. With the 2D card object, it does cast shadows, but the texture is too dark. At this time, I am assuming its due to an object flaw. Even with diffuse color set to 1, the object renders too dark (see below).
The other 2D object I have shows the image perfectly, but does not cast shadows.
Below, I've attached a sample image of the "darkness" and the 2D card object, plus the test image I've been using.
The main reason I am pursuing using a 2D object, is for extremely massive populations. I've been working on one image that consistently crashes, due to sheer volume of 3D populations. In certain cases, the difference between 2D and 3D is negligible, so I've decided to go this route in certain cases.