This is not entirely accurate. The provision in question can be found here:
https://www.legifrance.gouv.fr/conte...le/Code_33.pdf
The decision and their reasoning can be found here:ARTICLE 227-24 (Ordinance no. 2000-916 of 19 September 2000 Article 3 Official Journal of 22 September 2000 in force 1 January 2002)
The manufacture, transport, distribution by whatever means and however supported, of a message bearing a pornographic or violent character or a character seriously violating human dignity, or the trafficking in such a message, is punished by three years' imprisonment and a fine of €75,000, where the message may be seen or perceived by a
minor.
Where the offences under the present article are committed through the press or by broadcasting, the specific legal provisions governing those matters are applicable to define the persons who are responsible.
http://www.europarl.europa.eu/sides/...ML&language=EN
You obviously can't persuasively argue that Le Pen was deliberately inciting jihadist violence. It may be possible to argue that these images have the unintended effect of encouraging extremists to take the final step into outright terrorism (one reason why French news sources have of late been reluctant to publish images of terrorists) but that's not a matter for the law to decide. The argument being made is that the dissemination of these images violates human dignity according to the standards of the French criminal code.