This site has an article and small photo of a le Prieur rocket arrangement, they appear to be on the inside of the outboard struts.
http://www.firstworldwar.com/atoz/leprieur.htm
This is a bigger version of the same image:
"Invented by Frenchman Lieutenant Yves Le Prieur the solid-fuel stick-guided rocket was first deployed during the Battle of Verdun in April 1916 when four rockets were attached to each strut of a Nieuport fighter aircraft and fired through steel tubes."