Q: Centering table with 4 images in beamer I am trying to center a table containing 4 images and a foot note in beamer (the lines are only for debug). This is the code I am using:
\documentclass[]{beamer}

\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc} % acentos
\usepackage{listings} % incluir código
\usepackage{caption}  % titulos para los listings
\usepackage{color}
\begin{document}
\begin{frame}
    \begin{centering}
        \begin{tabular}{ | r | l | }
            \includegraphics[width=0.325\textwidth]{img/g2} &
            \includegraphics[width=0.325\textwidth]{img/g2Isodata} \\
            \includegraphics[width=0.325\textwidth]{img/g2Huang} &
            \includegraphics[width=0.325\textwidth]{img/g2Default} \\
            \multicolumn{2}{p{\dimexpr\textwidth-\tabcolsep\relax}}{\tiny \center Imagen binarizada mediante el algoritmo Isodata (arriba a la derecha), el algoritmo de Huang (abajo a la izquierda) y el algoritmo por defecto (abajo a la derecha)} \\

        \end{tabular}
    \end{centering}
\end{frame}
\end{document}

This is however the result I am getting:

I dont understand why it behaves that way. How can I properly center that table?

A: It's the \multicolumn command, i.e. the parbox in it -- the OP used basically the whole \textwidth for the width of that \multicolumn.
Since both images have 0.325\textwidth, it's basically sufficient to use 0.65\textwidth for the \parbox. (Not taking into account any other \tabcolsep)
I added some 'improvements' -- remove demo from the options to the beamer class for production run!
\documentclass[demo]{beamer}

\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc} % acentos
\usepackage{calc}%
\usepackage{listings} % incluir código
\usepackage{caption}  % titulos para los listings
\usepackage{color}
\begin{document}
\begin{frame}
  \begin{center}
    \begin{tabular}{| c | c |}
      \includegraphics[width=0.325\textwidth]{img/g2} &
      \includegraphics[width=0.325\textwidth]{img/g2Isodata} \tabularnewline
      \includegraphics[width=0.325\textwidth]{img/g2Huang} &
      \includegraphics[width=0.325\textwidth]{img/g2Default} \tabularnewline
      \multicolumn{2}{p{0.65\textwidth-\tabcolsep}}{\tiny \centering Imagen binarizada mediante el algoritmo Isodata (arriba a la derecha), el algoritmo de Huang (abajo a la izquierda) y el algoritmo por defecto (abajo a la derecha)}
    \end{tabular}
\end{center}
\end{frame}
\end{document}


