\node[minimum height=\paperheight, minimum width=\paperwidth](centre) {};
% placement des logos en bas à gauche
\matrix[above right=4mm, ampersand replacement=\&] at (centre.south west){
+ \node[anchor=center]{\includegraphics[width=7mm]{ubfc.jpeg}}; \&
\node[anchor=center]{\includegraphics[width=7mm]{logo-UFC.jpg}}; \&
\node[anchor=center]{\includegraphics[width=6mm]{logo-CNRS.jpg}}; \&
\node[anchor=center]{\includegraphics[width=13mm]{logo-ENSMM.jpg}}; \&