French law regarding image is a bit complicated... If you intend to use your pictures just for a website, though, it's generally admitted that all you need is to ask permission to the cosplayer to put them online.
When you put them online, you should announce it on the forum where French cosplayers meet (http:///www.cosplayforum.com/
) ; there's a section dedicated to photos, and after each con a topic is created for this kind of annoucement. Expect two things :
* that cosplayers use your pictures for their personal website, on cosplay.com, and whatever they please.
* that some ask you to put some pictures offline because they don't like how they look on it. Even if you don't risk a lawsuit, it's polite to do it
Epitanime itself does not have special rules per se, it's a very relaxed event. One thing though : there is an area for cosplayers next to the changing rooms. You can go there if you want, BUT don't take pictures behind the back of the photographers there. The booth is for the official photographers of the event, and only them. You can take pictures elsewhere in the room though
(see my other post in the photography forum for more things)