Several different ways to do this. You could stretch a sheet of plastic and have someone press their face into it. Doing it in photoshop. you can take and face and then create a displacement map to bend the shape of the wall then appling the image as a B&W in overlay mode to give the shaped tones to the wall. Such as the image below of the face on the path. The other option would be to get a 3D face like those in Poser, bring it into PS and convert it to a depthmap and then blend the depthmap with plain backgroud by blurring the edges, such as the other image attached of the face coming out of the building.