The presumption of authorship (L113.1 of French IP code) only applies when the communication of a work has been done in an unequivocal manner including an intent from the person behind the communication to present himself/herself as the author.
The posting of a picture on the internet under a pseudonym does not presume that this person is the author of the photograph says French TGI on 20th December 2012.
See full decision on Legalis: http://www.legalis.net/spip.php?page=jurisprudence-decision&id_article=3578