\documentclass[portrait]{sciposter}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{multicol}
\usepackage{graphicx}
\usepackage[english]{babel} 
\usepackage{fancybox}
 

\begin{document}
 
\includegraphics[width=\textwidth,]{multiplo.png} 

\begin{minipage}[c]{\linewidth}   

\begin{minipage}[c]{0.25\linewidth}   
\begin{LARGE} 
% exemple
Chaire d'analyse \\ et simulation num\'eriques \\
\end{LARGE} 
\end{minipage}  \;
\begin{minipage}[c]{0.65\linewidth}    
\begin{LARGE}
% exemple
Simulation num\'erique du mouvement des glaciers\\
\end{LARGE}
\end{minipage}  

\end{minipage}
 
\end{document}

 

  