usepackage{subfig}

renewcommandthesubfigure{}
setbeamertemplate{navigation symbols}{}
beamertemplatenavigationsymbolsempty

begin{figure}
begin{center}
mbox{subfloat[{tiny MRI scanner}]{includegraphics[width=0.5textwidth]{mri-scanner.jpg}}qquad
  subfloat[{tiny MRI of the knee}]{includegraphics[width=0.3textwidth]{knee-mri.jpg}}}
end{center}
end{figure}