I must say to the list that the code that solves my problem is given in Eric Fraga's tutorial:

#+latex_header: \mode<beamer>{\usetheme{CambridgeUS}\usepackage{beamer_udl_theme}}

where the last \usepackage refers to a personal customization of CambridgeUS theme.

Now I meet a new problem : what is the org-mode code to export this Latex code that customizes the frame of the title page? :

\begin{frame}
\titlepage
\begin{center}
\includegraphics[height=1cm,width=1cm]{cnrs_noir}~~
\includegraphics[height=1cm,width=1cm]{logo}~
\includegraphics[height=1cm,width=1cm]{logo-udl}
\end{center}
\end{frame}

Best wishes

Jo.