It does work for most birds, when they have no other pigeons to interact with.
Remember they have to get used to seeing the image daily, so at first both sexes may see it as an intruder and attack.
Give the bird some time to get used to it, and eventually your bird may respond with loud roo-koos and a dance,-thinking that the image could be a future mate. This would indicate a male.
The hen would be more docile, and eventually might respond to the image by cooing and make the "come hither" move, accepting the bird. I would also be more inclined to think your bird is a hen, if she has little response to the image.
Nothing works better then introducing the bird to a coop full of pigeons, within seconds you will know.