Hi Brian,
Think of the gray values like a measurement. From the camera to a certain point in space the values express close to distance OR from black to white. Like you have for an 8bit/c image the values 0-255 for a channel.
The gray map/texture is a translation from distances. like zero meter would be black and 100 meter would be white, then a middle gray should be given between zero and 100 meter. (If used in a gamma free image…but that is perhaps not important while starting to explain that technique).
Important is to understand that “we” use here images to store and use values. In terms of your image, the space is perhaps only a few centimeters or some inches. To get the most out of the gray map, you need to “normalize” this. Which means nothing else that the ears would be a dark gray, the cheeks a middle gray, and the tip of the nose a bright grey. These Values can be then used to move the clones (in the nail board the nails) back and fore.
The Shader Effector can take from a texture these values and based on the parameters you set into it the nails move accordingly. Please understand the Shader effector only as an option to get a single pixel value for each object, it does not work as a shader for a single clone (a common misconception)
You picture might work to the degree that the nails are moving—somehow—but there is no option to get from this image anything that would resemble the result that you would get in reality, pushing your face into this kind of object.
Does it makes more sense now? Let me know.
All the best
Sassi
This is how your image should look like to get the effect: