\documentclass{beamer}
\usepackage{tikz}   
\begin{document}

\begin{frame}
\begin{center}
\begin{tikzpicture}
  \node (img1) {\includegraphics[height=3cm]{bb-crop}};
  \pause
  \node (img2) at (img1.south east) {\includegraphics[height=3cm]{bb-crop}};
  \pause
  \node (img3) at (img2.south west) [yshift=1cm] {\includegraphics[height=3cm]{bb-crop}};
\end{tikzpicture}
\end{center}
\end{frame}

\end{document}