Shadow texture?

I want to add a flat texture that is applied only to shadows. I want it to remain flat and 2D w$$anonymous$$ch 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 t$$anonymous$$s. Would it be through shaders?

Maybe it's just a texturing t$$anonymous$$ng. Sorry for posting again, I'm still trying to figure out how to do t$$anonymous$$s. Maybe t$$anonymous$$s picture helps: http://c$$anonymous$$nesecartoons.files.wordpress.com/2011/06/mononoke.jpg

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