Reference:No Image Object Modifier

Syntax:

OBJECT {
  [OBJECT_ITEMS...]
  no_image
  }

This keyword is very similar in usage and function to the no_shadow keyword, and control an object's visibility.
You can use any combination of no_image, no_reflection and no_shadow with your object.

When no_image is used, the object will not be seen by the camera, either directly or through transparent/refractive objects. However, it will still cast shadows, and show up in reflections (unless no_reflection and/or no_shadow is used also).

Using these three keywords you can produce interesting effects like a sphere casting a rectangular shadow, a cube that shows up as a cone in mirrors, etc.