How to overlap images in a beamer slide?

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}

The second and third images are placed on a corner of the previous image, the third also shifted a little upward, the reason being that with the images I used there was some whitespace between them. You could of course place the images at specific coordinates, and not relative to each other like here.