Shadow texture?

I want to add a flat texture that is applied only to shadows. I want it to remain flat and 2D which I'm guessing means I need to somehow have the texture always orient itself towards the camera? I'm relatively new to Unity so I'm stumped on how to do this. Would it be through shaders?

Maybe it's just a texturing thing. Sorry for posting again, I'm still trying to figure out how to do this. Maybe this picture helps: http://chinesecartoons.files.wordpress.com/2011/06/mononoke.jpg

Just looking for a way to apply flat 2d textures that stay oriented on 3d objects.