\begin{frame}
    \frametitle{Zeitstempel}
    \begin{center}
      \only<1>{
        \includegraphics[width=0.9\textwidth]{img/punch_1.png}
        \\ \hfill \small Alan, Microblogging
      }

      \only<2>{
        \includegraphics[width=0.9\textwidth]{img/punch_2.png}
        \\ \hfill \small Bob, Microblogging
      }

      \only<3>{
        \includegraphics[width=0.9\textwidth]{img/punch_3.png}
        \\ \hfill \small Charlie, Github
      }
    \end{center}
\end{frame}