Q:

Pictures of equal size have different position when exchanged by the only command in a beamer presentation

I want to have the same basic slide with different pictures. For this I tried to use the only command. My minimal working example is
\documentclass[12pt]{beamer}
\usetheme{Goettingen}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{graphicx}
\begin{document}

\begin{frame}{My Example Frame}
\begin{itemize}
\item Text 1
\item Text 2
\item Text 3
\end{itemize}

\begin{columns}[C]
\column{.5\textwidth}
\begin{enumerate}
\item First
\item Second
\item Third
\end{enumerate}
\column{.5\textwidth}
\only<1>{
\includegraphics[width=\textwidth]{Your_Picture}}
\only<2>{
\includegraphics[width=\textwidth]{Your_Picture}}
\only<3>{
\includegraphics[width=\textwidth]{Your_Picture}}
\end{columns}

\end{frame}

\end{document}

The goal is to have the pictures, which all have the exact same size, always in the same position. 
Between the first and the second slide the position changes.
Any ideas or explanations?

A:

Use \includegraphics<1>{....} or \only<1>{%
\includegraphics{...}}% Note the % at the end -- omitting leads to spurious whitespace causing the image to be shifted!
The \only<...>{...} version might be useful if there are more things to do than just to provide another image!
\begin{column}[C] is wrong, as far as I know!
\documentclass[12pt,demo]{beamer}
\usetheme{Goettingen}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
%\usepackage{graphicx}
\begin{document}

\begin{frame}{My Example Frame}
\begin{itemize}
\item Text 1
\item Text 2
\item Text 3
\end{itemize}

\begin{columns}
\column{.5\textwidth}
\begin{enumerate}
\item First
\item Second
\item Third
\end{enumerate}
\column{.5\textwidth}
\only<1>{%
\includegraphics[width=\textwidth]{Your_Picture}}%
\only<2>{%
\includegraphics[width=\textwidth]{Your_Picture}}%
\only<3>{%
\includegraphics[width=\textwidth]{Your_Picture}}%
\end{columns}

\end{frame}

\end{document}

A:

Never ignore errors when compiling. Substituting example-image for Your_Picture, I immediately get a compilation error complaining about the C in \begin{columns}. So let's change that to start with:
\begin{columns}

Minimising the example a bit:
\documentclass{beamer}
\begin{document}
  \begin{frame}{My Example Frame}
    \begin{columns}%
      \begin{column}{.5\textwidth}
        \begin{enumerate}
          \item First
          \item Second
          \item Third
        \end{enumerate}
      \end{column}%
      \begin{column}{.5\textwidth}
      \only<1>{%
        \includegraphics[width=\linewidth]{example-image}}
      \only<2>{%
        \includegraphics[width=\linewidth]{example-image}}
      \end{column}%
    \end{columns}
  \end{frame}
\end{document}

reproduces the issue. This makes it easier to see what is happening. A line break is a space. So, on the first slide, we have
        \includegraphics[width=\linewidth]{example-image}
        {}

which will be understood as the image followed by two spaces, which TeX will collapse into the image followed by a single space.
On the second slide, we have
        {}
        \includegraphics[width=\linewidth]{example-image}

which will be interpreted as a space followed by the image followed by a space. Since the image requires the full width of the column, it cannot be accommodated after the space and so a line break will be inserted before the image.
Commenting out the spaces by adding a % at the ends of the lines resolves the problem:
  \only<1>{%
    \includegraphics[width=\linewidth]{example-image}}%
  \only<2>{%
    \includegraphics[width=\linewidth]{example-image}}%

However, it is easier to use the overlay specification as an argument to \includegraphics:
        \includegraphics<1>[width=\linewidth]{example-image}%
        \includegraphics<2>[width=\linewidth]{example-image}%

Complete code for the minimised example above with fixes:
\documentclass{beamer}
\begin{document}
  \begin{frame}{My Example Frame}
    \begin{columns}%
      \begin{column}{.5\textwidth}
        \begin{enumerate}
          \item First
          \item Second
          \item Third
        \end{enumerate}
      \end{column}%
      \begin{column}{.5\textwidth}
        \includegraphics<1>[width=\linewidth]{example-image}%
        \includegraphics<2>[width=\linewidth]{example-image}%
      \end{column}%
    \end{columns}
  \end{frame}
\end{document}

