Graphic Jumps Up in Beamer

Use the \visible<2>{...} macro to make the second image reserve space on the first slide to achieve correct alignment, but only be visible on the second slide. For text you can also use\uncover<2>{...} to achieve a semi-transparent effect (if configured), but it doesn't seem to have an effect on included images.

\documentclass[serif,xcolor=dvipsnames]{beamer}
\mode
{
  \usetheme{Warsaw}
  \usecolortheme{default}
  \setbeamercovered{transparent}
  \setbeamertemplate{footline}[frame number]
}
\begin{document}
\begin{frame}[plain]
    \begin{center}
      \includegraphics[scale=.3]{1.png}

      \vspace{2cm}

      \visible<2>{\includegraphics[scale=.3]{2.png}}
    \end{center}
\end{frame}
\end{document}

if you want to center the images use the environment center
\PassOptionsToPackage{demo}{graphicx}
\documentclass{beamer}
\begin{document}
\begin{frame}{Title}
\begin{overprint}
  \begin{center}
      \includegraphics[height=2cm]{1}

      \vspace{2cm}
      \onslide<2>{\includegraphics[height=2cm]{2}}
     \end{center}
  \end{overprint}
\end{frame}
\end{document}