[TeX][Beamer] Graphics overlap (superposition)

A TikZ example:

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

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

\end{document}

http://tex.stackexchange.com/questions/34921/how-to-overlap-images-in-a-beamer-slide