I'm trying to center all images vertically and horizontally within a table cell.
Therefore I'm using the adjustbox packet. This is the code I use for my table:
\begin{table}[b]
\begin{center}
\begin{tabular}{ |c|c|c|c|c| }
\hline
\adjustbox{valign=m}{
\includegraphics[width=2cm,height=1cm,keepaspectratio]{./images/buttons/Windows-Buttons.png}
}
&
\adjustbox{valign=m}{
\includegraphics[width=2cm,height=1cm,keepaspectratio]{./images/buttons/MacOSX-Buttons.png}
}
&
[...]
&
\adjustbox{valign=m}{
\includegraphics[width=2cm,height=1cm,keepaspectratio]{./images/buttons/Android-Buttons.png}
}
\\[.8cm]
\hline
Windows 7 & Mac OS X & Gnome & iOS & Android \\
\hline
\end{tabular}
\end{center}
\vspace{-.5cm}
\caption{Buttons verschiedener Betriebssysteme}
\end{table}
If I attach adjustbox to every \includegraphics I don't get the desired output: 
If I remove only one \adjustbox (no matter which one), with following code:
%\adjustbox{valign=m}{
\includegraphics[width=2cm,height=1cm,keepaspectratio]{./images/buttons/MacOSX-Buttons.png}
%}
I get the desired result (well, some kind of ...):

I don't have to use \adjustbox. I just want the images to be vertically and horizontally centered.
Thank you for your help!

%on lines ending on{to avoid getting a space here from the line break, i.e. write\adjustbox{valign=m}{%. Note that with theexportoption ofadjustboxyou can usevalign=mdirectly as\includegraphicsoption. There is also a\adjustimagemacro. Thecenterenvironment should not be used insidetable. See Should I use center or centering for figures and tables? – Martin Scharrer Jan 04 '13 at 20:55