Centering Text Horizontally and Vertically in LaTeX

Question

I would like to produce the following:

      a       b
    xxxxx   xxxxx
 1  xxxxx   xxxxx
    xxxxx   xxxxx

    xxxxx   xxxxx
 2  xxxxx   xxxxx
    xxxxx   xxxxx

Where the blocks of 'x's are images, and 'a', 'b', '1' and '2' are text.

Here are my two attempts so far:

\begin{figure}
\begin{center}
\begin{tabular}{ccc}
 & a & b \\
1 & \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} &
    \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} \\
2 & \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} &
    \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} \\
\end{tabular}
\end{center}
\end{figure}

Which produces:

      a       b
    xxxxx   xxxxx
    xxxxx   xxxxx
 1  xxxxx   xxxxx

    xxxxx   xxxxx
    xxxxx   xxxxx
 2  xxxxx   xxxxx

And

\begin{figure}
\begin{center}
\begin{tabular}{m{1cm}m{6cm}m{6cm}}
 & a & b \\
1 & \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} &
    \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} \\
2 & \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} &
    \subfloat[]{\includegraphics[width=0.47\textwidth]{im.png}} \\
\end{tabular}
\end{center}
\end{figure}

Which produces:

    a       b
    xxxxx   xxxxx
 1  xxxxx   xxxxx
    xxxxx   xxxxx

    xxxxx   xxxxx
 2  xxxxx   xxxxx
    xxxxx   xxxxx

Answer 1

I use \dummyimage, because I have no im.png. Replace it with \includegraphics{im.png}.

\font\dummyfont = cmr10 at 100pt
\def\dummyimage{{\vbox to 100pt{\vfil\hbox to 100pt{\hfil\dummyfont A\hfil}\vfil}}}

\hfil\vbox{
\halign{&\hfil\ $\vcenter{\hbox{#}}$\strut \ \hfil\cr
&a&b\cr 
1&\dummyimage&\dummyimage\cr
2&\dummyimage&\dummyimage\cr
}}

Answer 2

You can make a new column type, or simply add >{\centering\arraybackslash} before m{6cm} for the two image columns.

For example:

\newcolumntype{C}{>{\centering\arraybackslash} m{6cm} }  %# New column type
\begin{tabular}{m{1cm}CC}                                %# Table with two of them
...

The > directive lets you basically inject the contained code before each entry in that column. We need the \arraybackslash to deal with incompatibility between the centering environment and the tabular environment. More info can be found here.