Please help us to define and evaluate the author's picture

[redbor]

The picture was signed by the artist because he painted it to be signed, like any author.
  The works are usually signed, painting, sculpture, theorem, poem, etc. Or do without a signature.
  A stretcher and canvas, as I understand it, before writing signed by the picture, or there is evidence of life, such as Aivazovsky,
  that - by writing "A ship in the bay .." Ivan Konstantinych unrolled the canvas and signed on a stretcher ..?
  Why Levitan on a tiny sketch in the corner scratch out his signature? not wrote in a big way at the back of the cardboard?
  By signing, what they have created, rather than something from what he is going to create. These things are simply marked signature.
  On the stretcher can be written by anyone, and the author as well. Is not it?
But the picture of the author only, and no one else. Because this is so? (not talking about fakes).